Mattkeen,

people who play in sessions a lot can be almost brainwashed into thinking all tunes are 32 bars of AABB, so if you've just played BBA the next one must be another A.

But with dances that's not always the case, and you have to remain alert and dare not let the "autopilot" take over: a typical Cotswold morris tune would be played ABB, and Cock of the North would be played ABAB etc for the Gay Gordons, AAB etc for the Bromsberrow Heath stick dance, and ABABBABBBABB for Eynsham Figure of Eight - easy to lose track of what's next if you get distracted.