_complete:96: bad math expression: operand expected at end of string in iTerm and Terminal on Tab kypresses
A basic shell error message consists of a comand, a colon, a operand/filename, a colon, and a message, like

command: operand/filename: error message


as an example

ls foo
ls: foo: No such file or directory


These error message may contain multiple commands and/or error messages, as an example

ls | fake_command
-ksh: fake_command: not found


and without a operand/filename. But they are usually not that difficult to digest. In your case, your shell completions are producing the error message.