Steam keeps validating
ref=4988-DHXV-7272 It describes what to do in each case. Steam games cannot disappear from your Steam account, unless you acquired them illegally or it was a timed offer ( Free Weekend or a Guest Pass ).
The only official place to buy Facepunch games ( Rust and Garry's Mod ) is the Steam Online Store ( ).
Thus it can go into a continuous loop between the game and it's expansion, and unless stopped by the user, or steam or whomever, will just continually invalidate what steam downloads for those files that failed validation on KBCR during it's validation of KBAP and visa versa. Download fresh copy of KBCR, and then validate it, it may or may not find failed files, depending if steam ran validation for KBAP after you installed KBCR.
Sometimes a game on Steam will non-stop update and validate itself, and when trying to launch it, it will update and fail with reason "Corrupt update files" or something similar.
This can be caused by your anti virus or similar software deleting the update files before steam has a chance of copying them to their intended location to update a game.
To fix this problem either wait for anti virus update that fixes the problem, or add the problematic file to your anti virus exceptions.
After that, validate the game on Steam to redownload the missing file. While it shows 0% all of the time, it actually downloads your subscribed Steam Workshop mods.