Had a few tries at programming a Jupiter Ace, which was basically a Sinclair ZX-81 but running compiled Forth instead of interpreted BASIC. It was blazingly fast compared to a ZX-81, but the "keyboard" was even worst and I think it took me about three hours to write something that wasn't much more than a Hello World program; at that point someone came in and bought the computer, so I had to leave it.

I also tried it on my TRS-80 a couple of years later. The author of the program proudly claimed that it was a completely transparent compiler, you could modify anything at any level down to single bytes. Needless to say the disk was copy protected and I didn't have a backup. The first time I tried to use it I somehow modified the f****g program so that it wouldn't load any more...