I was testing the carriage contraptions on a server, connected 2 carts with each other, one empty, one with some wool on top. I connected them with each other using the 'minecart coupling' tool. Then after bumping into one of the carts, the game crashed and kept crashing while attempting to log in, had to restore from backup to fix it.
This is with version 0.3 in an all the mods 6 server

crash-2020-12-21_00.33.27-client.txt

Might be fixed by e2437d1 judging by the commit message , not sure

Yup, this was addressed in more recent versions, sorry for the late notice!