I've not made all, but most of the mistakes I'm listing here. I didn't want to post any in-mapping mistakes, because I thought sharing some general ideas would be more helpful.
working on assets and any kind of art before getting a map to a playable state.
not doing any sort of design or testing. just jumping straight into the editor to build the level working from a few/single idea(s).
thinking an idea for a map is actually worth something/worth more than the actual work on a map.
working on several projects at the same time.
planning too big.
giving up on a map because new ideas seem better for a new map.
I've not made all, but most of the mistakes I'm listing here. I didn't want to post any in-mapping mistakes, because I thought sharing some general ideas would be more helpful.
-- (inmapping mistakes)