There are a number of things which can cause a patch to fail.
Anti-xxxx software is the leading issue with running an SSD being next. Running out of physical system memory can also cause it. The patcher has to load two copies of each file into memory. One is the original file, the other is the new file so checksums can generated to make sure the patch worked.
When we patch a file like stdshapes,res, it takes about 4GB of free system RAM to do so.
Like HiTech said, once it happens, the only way to fix it is to install a full copy of the game, over the top of your existing installation.