On IRC dhs227 notes:

If I press F1 it generate [[A, then press enter. fish complains fish: Tokenizer error: “Unexpected end of string, parenthesis do not match”
the problems are: a) it complain twice.
b) it does not clear the cmd line, which I think it should be.
c) in this case,fish likely "eats" the latest cmd line somehow.

Obfuscate also notes that:
That seems like a bug. The handling of [ is awkward in general as well: [[ errors, [[ errors, [[ is an ordinary token, as is [[.

Above all are reproduced on 20130606 build.