Recently, I saw a crossword clue: (5 letters) "They help you after a crash." After going through the list of EMTs, firemen, police officers, etc. I realized that the answer was "IT Guy." This seems to me, to be analogous to the phenomenon of functional fixedness, and I was wondering if there is some linguistic equivalent to functional fixedness?
I'm not sure whether this can explain your crossword experience, but perseveration is a fairly well-studied phenomenon in word production. Individuals with aphasia (a neurogenic language disorder) often exhibit perseveration errors in tasks like picture naming. For example, imagine trial N is a picture of a dog and the participant responds "dog" (correct) and trial N+1 is a picture of a car and the participant again responds "dog" (perseveration error). Stark (2007) provides a good historical review of theories of perseveration in aphasia, which generally propose either failure to inhibit the representation of the previous response or failure to sufficiently activate the representation of the new stimulus (and it is possible that both can happen: Fischer-Baum & Rapp, 2012). Like other speech errors, perseveration errors are also produced by typical (non-aphasic) adults, just much less frequently than in aphasia.
More general kinds of perseveration - inability to stop focusing on a particular topic - are associated with deficits of executive function (possibly related to frontal lobe dysfunction or damage) and a variety of psychopathologies, such as depression.
Fischer-Baum, S., & Rapp, B. (2012). Underlying cause(s) of letter perseveration errors. Neuropsychologia, 50(2), 305–318. doi:10.1016/j.neuropsychologia.2011.12.001
Stark, J. (2007). A review of classical accounts of verbal perseveration and their modern‐day relevance. Aphasiology, 21(10-11), 928–959. doi:10.1080/02687030701198239