I had written a rather lengthy response to a post, and put quite a bit of effort in my argument. When I submitted the reply I got a “language not allowed” error, and everything I wrote was lost.

I think an error when posting something should not result in the screen changing back and losing everything written. The error should be displayed and the user allowed to fix the problem or somehow allowed to save the post locally, e.g. by copying and pasting it somewhere else.