My Commercial Work
There are no known bugs in PAIN version 1.21, but there are some issues you should probably be aware of. If you do find a bug, you can send it to me, but I doubt an updated version will ever be released.

GLQuake - If you are using glquake.exe to run Quake, you will need to delete your entire GLQUAKE directory from your ID1 directory before running PAIN for the first time. Otherwise, you will get several distorted models and skins.

Other versions of Quake - I have not tested PAIN with any version of Quake that has been built since the engine became open-source. I have absolutely no idea if it will work with any of them, but as far as I know, it should. Feel free to let me know your experiences if you try this out.

PainX - I have absolutely NO affiliation with PainX. The author of PainX took the source code from a beta version of PAIN and turned it into something I never wanted PAIN to become, a "kitchen sink" mod.