Many developers assume that if you have plenty of free VAS in your process VirtualAlloc (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/memory/base/virtualalloc.asp) and VirtualAllocEx (http://msdn.microsoft.com/library/default.asp?url=/library/en-us/memory/base/virtualallocex.asp) calls with MEM_RESERVE parameter can't fail. This is not actually true. Remember that when allocating VAS region OS creates VAD (Virtual Address Descriptor). VAD manages VAS region, for more info read http://blogs.msdn.com/slavao/archive/2005/01/27/361678.aspx. When system is low on physical memory which includes RAM and swap file, VAD's allocation might fail. The failure will cause VirtualAlloc* call to fail. It means that if system low on physical memory VirtualAlloc* could fail. This becomes very important on 64 bit platforms. When running on 64 bit, developers might assume that reservation never fails - which is wrong.
Hope it was useful.