Mrs. Doubtfire sets herself on fire. When she jumps back and starts fighting the flames, it is obviously a stunt double as her face is longer.
When Stu calls Daniel a loser at the swimming pool, Mrs. Doubtfire then picks up a lime and throws it at Stu, hitting him on the back of his head. Robin Williams managed to hit Pierce Brosnan on the third take and the lime you see was made of foam rubber. Mrs. Doubtfire then tells Stu it was a 'run-by fruiting' - this line was improvised by Williams.