Can't load lump x, allocation of x bytes failed! Engine Error

Can't load lump x, allocation of x bytes failed! Engine Error
Question

I am encountering the error Can't load lump x, allocation of x bytes failed! Engine Error while playing a Source-engine game. What should I do?
Answer
Important:

It is highly recommended that you visit Windows Update to ensure that your system has all of the recent updates available. Updates for Windows can be downloaded here: Windows Update
Verify your Game Cache Files

This issue can also be the result of corruption in your game's cache files. Please see the Verifying Game Cache Files topic for more information on how you can correct this issue.


Windows Vista Users

This error may be related to the way in which Windows Vista handles memory allocation. If you are using Windows Vista, please see the Virtual address space usage in Windows game development topic at the Microsoft Help and Support center.