Crash when accessing Read This!
From DoomWiki.org
When trying to use The Ultimate Doom with the Final Doom executable, the engine will crash when selecting "Read This!" on the menu, with an error message stating W_GetNumForName: HELP not found!
This problem was also introduced with the special id Anthology version of Doom95.
Conversely, while using Doom II with The Ultimate Doom executable, pressing F1 will cause a crash with the message W_GetNumForName: HELP1 not found!
Originally, Doom had two help screen lumps, HELP2 and HELP1, while Doom II only had one, HELP. The Ultimate Doom removed HELP2 (the screen with the ordering information), and this change inadvertently caused HELP1 to be used even while running Doom II, which causes the crash. Final Doom attempted to fix this by substituting HELP1 with HELP, making The Ultimate Doom crash instead.