diff --git a/source/lib/status.cpp b/source/lib/status.cpp
index 88a94dcad6..a967868b42 100644
--- a/source/lib/status.cpp
+++ b/source/lib/status.cpp
@@ -130,6 +130,7 @@ STATUS_DEFINE(ERR, TIMED_OUT, L"Timed out", -1);
 STATUS_DEFINE(ERR, REENTERED, L"Single-call function was reentered", -1);
 STATUS_DEFINE(ERR, CORRUPTED, L"File/memory data is corrupted", -1);
 STATUS_DEFINE(ERR, VERSION,   L"Version mismatch", -1);
+STATUS_DEFINE(ERR, ABORTED,   L"Operation aborted", -1);
 
 STATUS_DEFINE(ERR, INVALID_PARAM, L"Invalid function argument", EINVAL);
 STATUS_DEFINE(ERR, INVALID_HANDLE, L"Invalid Handle (argument)", -1);
diff --git a/source/lib/status.h b/source/lib/status.h
index 9ef5461dbd..12967af031 100644
--- a/source/lib/status.h
+++ b/source/lib/status.h
@@ -251,7 +251,7 @@ extern Status StatusFromErrno();
 	{\
 		const Status status_ = (expression);\
 		if(status_ < 0)\
-		DEBUG_WARN_ERR(status_);\
+			DEBUG_WARN_ERR(status_);\
 	}\
 	while(0)
 
@@ -375,6 +375,7 @@ namespace ERR
 	const Status REENTERED = -100012;
 	const Status CORRUPTED = -100013;
 	const Status VERSION   = -100014;
+	const Status ABORTED   = -100015;
 
 	// function arguments
 	const Status INVALID_PARAM  = -100020;
diff --git a/source/lib/sysdep/os/win/mahaf.cpp b/source/lib/sysdep/os/win/mahaf.cpp
index 42550875d1..d56e6232d4 100644
--- a/source/lib/sysdep/os/win/mahaf.cpp
+++ b/source/lib/sysdep/os/win/mahaf.cpp
@@ -251,7 +251,11 @@ static void StartDriver(const OsPath& driverPathname)
 {
 	const SC_HANDLE hSCM = OpenServiceControlManager();
 	if(!hSCM)
+	{
+		ENSURE(GetLastError() == ERROR_ACCESS_DENIED);
+		SetLastError(0);
 		return;
+	}
 
 	SC_HANDLE hService = OpenServiceW(hSCM, AKEN_NAME, SERVICE_ALL_ACCESS);
 
@@ -328,6 +332,8 @@ static OsPath DriverPathname()
 
 static Status Init()
 {
+	WinScopedPreserveLastError s;
+
 	if(wutil_HasCommandLineArgument(L"-wNoMahaf"))
 		return ERR::NOT_SUPPORTED;	// NOWARN
 
@@ -340,7 +346,11 @@ static Status Init()
 		const DWORD shareMode = 0;
 		hAken = CreateFileW(L"\\\\.\\Aken", GENERIC_READ, shareMode, 0, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
 		if(hAken == INVALID_HANDLE_VALUE)
+		{
+			ENSURE(GetLastError() == ERROR_FILE_NOT_FOUND);
+			SetLastError(0);
 			return ERR::INVALID_HANDLE;	// NOWARN (happens often due to security restrictions)
+		}
 	}
 
 	return INFO::OK;
diff --git a/source/lib/sysdep/os/win/wdir_watch.cpp b/source/lib/sysdep/os/win/wdir_watch.cpp
index 371ffc029e..7093c45a66 100644
--- a/source/lib/sysdep/os/win/wdir_watch.cpp
+++ b/source/lib/sysdep/os/win/wdir_watch.cpp
@@ -333,16 +333,12 @@ public:
 
 	Status Poll(DirWatchNotifications& notifications)
 	{
+POLL_AGAIN:
 		DWORD bytesTransferred; ULONG_PTR key; OVERLAPPED* ovl;
-		for(;;)	// skip notifications of canceled watches
-		{
-			const Status ret = PollCompletionPort(hIOCP, 0, bytesTransferred, key, ovl);
-			if(ret == INFO::OK)
-				break;
-			if(GetLastError() == ERROR_OPERATION_ABORTED)
-				continue;		// watch was canceled - ignore
-			return ret;
-		}
+		const Status ret = PollCompletionPort(hIOCP, 0, bytesTransferred, key, ovl);
+		if(ret == ERR::ABORTED)	// watch was canceled
+			goto POLL_AGAIN;
+		RETURN_STATUS_IF_ERR(ret);
 
 		DirWatchRequest* request = (DirWatchRequest*)key;
 		request->RetrieveNotifications(notifications);
diff --git a/source/lib/sysdep/os/win/whrt/whrt.cpp b/source/lib/sysdep/os/win/whrt/whrt.cpp
index ef5cd495f0..12334c38ea 100644
--- a/source/lib/sysdep/os/win/whrt/whrt.cpp
+++ b/source/lib/sysdep/os/win/whrt/whrt.cpp
@@ -30,7 +30,7 @@
 #include <process.h>	// _beginthreadex
 
 #include "lib/sysdep/cpu.h"
-#include "lib/sysdep/os/win/win.h"
+#include "lib/sysdep/os/win/wutil.h"
 #include "lib/sysdep/os/win/winit.h"
 #include "lib/sysdep/acpi.h"
 #include "lib/bits.h"
@@ -261,6 +261,8 @@ static unsigned __stdcall UpdateThread(void* UNUSED(data))
 
 static inline Status InitUpdateThread()
 {
+	WinScopedPreserveLastError s;	// CreateEvent
+
 	// make sure our interval isn't too long
 	// (counterBits can be 64 => Bit() would overflow => calculate period/2)
 	const double period_2 = Bit<u64>(counterBits-1) / nominalFrequency;
diff --git a/source/lib/sysdep/os/win/wiocp.cpp b/source/lib/sysdep/os/win/wiocp.cpp
index 4696407cac..f489c54e92 100644
--- a/source/lib/sysdep/os/win/wiocp.cpp
+++ b/source/lib/sysdep/os/win/wiocp.cpp
@@ -20,13 +20,15 @@ Status PollCompletionPort(HANDLE hIOCP, DWORD timeout, DWORD& bytesTransferred,
 	if(hIOCP == 0)
 		return ERR::INVALID_HANDLE;	// NOWARN (happens if called before the first Attach)
 
+	WinScopedPreserveLastError s;
+
 	bytesTransferred = 0;
 	key = 0;
 	ovl = 0;
 	if(GetQueuedCompletionStatus(hIOCP, &bytesTransferred, &key, &ovl, timeout))
 		return INFO::OK;
-	if(GetLastError() == WAIT_TIMEOUT)
-		return ERR::AGAIN;	// NOWARN (nothing pending)
-	else
-		return ERR::FAIL;	// NOWARN (let caller decide what to do)
+	const Status ret = StatusFromWin();
+	if(ret == ERR::AGAIN || ret == ERR::ABORTED)	// avoid polluting last error
+		SetLastError(0);
+	return StatusFromWin();	// NOWARN (let caller decide what to do)
 }
diff --git a/source/lib/sysdep/os/win/wposix/wfilesystem.cpp b/source/lib/sysdep/os/win/wposix/wfilesystem.cpp
index bdd0cbdad3..68ae5b2d36 100644
--- a/source/lib/sysdep/os/win/wposix/wfilesystem.cpp
+++ b/source/lib/sysdep/os/win/wposix/wfilesystem.cpp
@@ -156,8 +156,10 @@ struct wdirent* wreaddir(WDIR* d)
 		{
 			if(!FindNextFileW(d->hFind, &d->findData))
 			{
-				if(GetLastError() != ERROR_NO_MORE_FILES)	// an actual error occurred
-					WARN_IF_ERR(StatusFromWin());
+				if(GetLastError() == ERROR_NO_MORE_FILES)
+					SetLastError(0);
+				else	// unexpected error
+					DEBUG_WARN_ERR(StatusFromWin());
 				return 0;	// end of directory or error
 			}
 		}
diff --git a/source/lib/sysdep/os/win/wsdl.cpp b/source/lib/sysdep/os/win/wsdl.cpp
index b106e1d65b..13c82d0da8 100644
--- a/source/lib/sysdep/os/win/wsdl.cpp
+++ b/source/lib/sysdep/os/win/wsdl.cpp
@@ -1485,6 +1485,8 @@ static void RedirectStdout()
 	if(wutil_IsValidHandle(GetStdHandle(STD_OUTPUT_HANDLE)))
 		return;
 
+	WinScopedPreserveLastError s;	// ChangeExtension
+
 	// this code may be included in multiple executables sharing the same
 	// directory, so include the executable's name in the filename. use its
 	// full path since the current directory is unreliable.
@@ -1495,9 +1497,12 @@ static void RedirectStdout()
 	// that means stdout isn't associated with a lowio handle; _close is
  	// called with fd = -1. oh well, there's nothing we can do.
  	FILE* f = 0;
-	// (return value ignored - it indicates 'file already exists' even
-	// if f is valid)
-	(void)_wfreopen_s(&f, OsString(pathname).c_str(), L"wt", stdout);
+	errno_t ret = _wfreopen_s(&f, OsString(pathname).c_str(), L"wt", stdout);
+	// (ignore return value - it might indicate 'file already exists' even
+	// if f is valid, which is what actually counts)
+	UNUSED2(ret);
+	if(GetLastError() == ERROR_ALREADY_EXISTS)
+		SetLastError(0);
 	// executable directory (probably Program Files) is read-only for
 	// non-Administrators. we can't pick another directory because
 	// ah_log_dir might not be valid until the app's init has run,
diff --git a/source/lib/sysdep/os/win/wsysdep.cpp b/source/lib/sysdep/os/win/wsysdep.cpp
index 41fe85d7a4..a8e8e8ef98 100644
--- a/source/lib/sysdep/os/win/wsysdep.cpp
+++ b/source/lib/sysdep/os/win/wsysdep.cpp
@@ -390,6 +390,7 @@ Status sys_get_module_filename(void* addr, OsPath& pathname)
 
 OsPath sys_ExecutablePathname()
 {
+	WinScopedPreserveLastError s;
 	OsPath pathname;
 	ENSURE(GetModulePathname(0, pathname) == INFO::OK);
 	return pathname;
diff --git a/source/lib/sysdep/os/win/wutil.cpp b/source/lib/sysdep/os/win/wutil.cpp
index a8cce82d6e..2b1dc50035 100644
--- a/source/lib/sysdep/os/win/wutil.cpp
+++ b/source/lib/sysdep/os/win/wutil.cpp
@@ -145,7 +145,10 @@ Status StatusFromWin()
 	case ERROR_PROC_NOT_FOUND:
 		return ERR::NO_SYS;
 	case ERROR_BUSY:
+	case WAIT_TIMEOUT:
 		return ERR::AGAIN;
+	case ERROR_OPERATION_ABORTED:
+		return ERR::ABORTED;
 	case ERROR_FILE_NOT_FOUND:
 		return ERR::VFS_FILE_NOT_FOUND;
 	case ERROR_PATH_NOT_FOUND:
@@ -289,6 +292,8 @@ static void GetDirectories()
 		wchar_t path[MAX_PATH];	// mandated by SHGetFolderPathW
 		const HRESULT ret = SHGetFolderPathW(hwnd, CSIDL_APPDATA, token, 0, path);
 		ENSURE(SUCCEEDED(ret));
+		if(GetLastError() == ERROR_NO_TOKEN)	// avoid polluting last error
+			SetLastError(0);
 		appdataPath = new(wutil_Allocate(sizeof(OsPath))) OsPath(path);
 	}
 }
@@ -340,6 +345,14 @@ static void FreeUser32Dll()
 
 static void EnableLowFragmentationHeap()
 {
+	if(IsDebuggerPresent())
+	{
+		// and the debug heap isn't explicitly disabled,
+		char* var = getenv("_NO_DEBUG_HEAP");
+		if(!var || var[0] != '1')
+			return;	// we can't enable the LFH
+	}
+
 #if WINVER >= 0x0501
 	WUTIL_FUNC(pHeapSetInformation, BOOL, (HANDLE, HEAP_INFORMATION_CLASS, void*, size_t));
 	WUTIL_IMPORT_KERNEL32(HeapSetInformation, pHeapSetInformation);