Duh
But yes,
http://www.dxalpha.co.uk/kb/multiplayer ... tch-errors
You see a popular server, 8/10 slots full, 80 ping, playing on DXMP_Cathedral. You attempt to join and you get the old MisMatch error (I'll explain why later). Now the best way to fix this is:
1. Close down all instances of Deus Ex.
2. Locate your Maps folder, generally in MyComputer>C:>DeusEx>Maps.
3. Locate DXMP_Cathedral, most likely near the bottom.
4. Either Drag this onto your desktop or some other place, or delete it.
5. Now attempt to re-join the server. You should be able to join without error.
If you still get the MisMatch error, and you have definitely taken DXMP_Cathedral away from your Deus Ex folder, it may be required that you purge your cache. The cache, in brief, stores all of the Downloads you have taken from servers, so Gun Mods, Maps, Anti-Cheating Mods, etc. Either delete all of that or take about 3-5 out at a time, then try to re-join the server. I won't go into detail on this, as I will probably just confuse you.