During a quiet period while directing, my thought wandered to OLOOTS (as you do), especially as at the Club TD course we were recommended to learn by heart what to say when such a situation arises.
South (after an involved transfer sequence that does not deserve to be shown) becomes the presumed declarer in e.g. 4 Spades and North (the player with the spades) turns to East and says "Your lead".
East leads and South starts to put down 'dummy' exposing at least one card - at this point West says "Hold on! Isn't South declarer?". This is agreed and you are called. Do you?
a) Give South the 5 options? (Law 54B, Law 54D and law 50D)
b) Rule that as South has started to display his hand he has accepted the lead, must become dummy and North is now the declarer. (Law 54A)
c) Something else?
Whilst sorting this out, it then turns out that NS gave incorrect information to EW during the Auction.
d) Can the TD give EW the right to change the last pass (assuming they might have done something different had they known the correct informaiton), and thus let the auction recommence.
e) If he can't, can the opening lead be changed?