When creating a MOD for a source game using a more recent version of Visual Studio (2010 or 2012), you might encounter a problem described in the image below.
To fix this problem, right click on server_hl2mp (or client if that's where the problem occurs!). Next click on properties -> Configuration properties -> C/C++ -> Preprocessor.
On the first line named "Preprocessor definitions" add the following line: "_ITERATOR_DEBUG_LEVEL=0" (without quotation marks). This fixed the problem for me, alternate solutions can be found on steamforums and other general coding forums such as stackoverflow. Depending on your version of visual studio, this solution might not work for you but it's surely worth a try as it only takes you a few seconds.