On Friday I logged out of my big dig world and went to sleep. On Saturday I did not touch big dig instead I downloaded tekkit classic with the launcher. On Monday I decided I wanted to play some more big dig but when I started up the game ssp and smp they both started giving me the id mismatch warning for thermal expansion. The sort of error like thermal expansion was completely removed from the game when it clearly was still in the mod folder. The only reason I mentioned tekkit classic is because that's the only interaction at all I had with minecraft from logging out of big dig on friday then trying to log back into big dig on Monday. Probably irrelevant but but then I've never had problems with a mod id mismatching on its own. Sorry if this confuses you more but I to am confused about the problem lol. Thanks for the reply.
Oh and my server boots up fine it's just my client and my server think thermal expansion is missing so that's why my server won't start up.