In the first X-Men film, during the Statue of Liberty fight, Cyclops, Wolverine, Jean Grey, and Storm are ambushed by Mystique and Toad, who had both been alerted earlier of the X-Men's arrival. Within less than 5 minutes, Wolverine is stuck fighting Mystique in another section of the Statue, Cyclops is locked in a display case, Storm is locked in an elevator shaft, and Jean is slimed by Toad.
I am curious, however, if the first X-Men film is the only instance of Toad sliming her, or if maybe it was shown again in one of the animated series or comics?