By default, completion in the Debug Console requires pressing <tab> to complete. Because I'm used to pressing <enter> I often send incomplete/invalid expressions. However these are still recorded to the history and then show in future invocations (which means the "top" entry is not the complete variable name that I want, but a previous mistyping):

I think it would be better if expressions are only recorded to the history if they evaluate successfully and do not return an error ("success":false).