- Check your common proxy for that import
- Make sure a common class does not access a method statically of a class which imports client only stuff
These are cases that can happen quickly and without intention (through human error or sometimes when IDEs do stuff) and I have seen them a couple of times already.
Generally everyone will need to have every mod, there are some server-only mods and some client-only mods, provided they are properly written they will auto-disable if they are on the wrong side, if they aren't then they'll crash and the crash report should make it obvious.
Custom maps should work fine as long as you have at least the mods from the save.
if someone made a mod that put 1.13 biomes and mobs into 1.12.2 That would be great. The best of both worlds. But that won't happen, 1.13 is just too different from 1.12.2, and while I'm having tons of fun with it I really miss my mods .