The " switch" is called the " gear flap inhibit switch" in checklists etc. However in every 737 I have operated it was TWO separate guarded switches, one for gear one for flaps specifically to avoid this possibility. Seems more likely someone flipped them both up, or the wrong one ( if the subsequent landing used normal 30 0r 40 flap there would be no warning) Someone else then either didn't monitor the switching or didn't understand the implications ? Just a guess, but I can see the mechanism for it going that way, particularly if the crm style was "single crew hands all over the place if I want your advice I'll ask for it" which may or may not have been the case.