What does Romeo say that precipitates Mercutio's famous Queen Mab Speech?
They are on their way to crashing the Capulet party and Romeo mentions that, "I dream'd a dream tonight". This sets Romeo onto his Mab tirade.
MERCUTIO"S famous "Queen Mab" speech is movtivated by Romeo's stubborn refusal to join in the fun that Benvolio and Mercutio have planned. In Franco Zeferelli's often-shown film version, Mercutio delivers the speech as though he were afflicted with some sort of deep personal hysteria. This delivery makes for an interesting effect, but it obscures the fact that Mercutio has a very clear main point, which is that Romeo is being silly.
oops, I meant Romeo's dream statement sets Mercutio onto his "Mab" tirade.