What is a 'Fatal Error' in the context of Sonic fan games like Sonic vs. Lord X?

A 'Fatal Error' in Sonic fan games, particularly those using the Sonic Mega Collection or Sonic Adventure engine, typically indicates a critical programming error that prevents the game from continuing. It often results in a crash to the desktop and can be caused by issues like corrupted data, incorrect memory access, or conflicts within the game's code. In Sonic vs. Lord X, it's a common issue due to the extensive modifications and additions to the base game.