I would certainly consider this a "gotcha" because it's unexpected that map both deviates from the norm and the language makes it so easy to misuse and hide tha

I would certainly consider this a "gotcha" because it's unexpected that map both deviates from the norm and the language makes it so easy to misuse and hide that misuse. This reminds me of "array set" in Tcl (a language many probably haven't used in a long time -- or ever). Unlike the normal "set" command in Tcl, which overwrites the full value of the variable, "array set" is actually a merge operation -- it merges in new keys and doesn't get rid of anything. I've seen experienced programmers use "array set" in a loop, leading to awful bugs. This too could be dismissed by saying that they "don't even know how array set works," but actually I place the blame on the language for making it so easy to misuse.