They often do. Introverts don't like to express their feelings and emotions. In situations where they feel that people would notice their introversion they pretend to be extroverts to avoid being noticed for their introversion.
This is a defence mechanism used by some introverts.
Introverts often mention episodes where they have been extroverts momentarily. Their over cautious behaviour about self expression makes them feel that they have been behaving like extroverts. Too much repression can cause venting out of emotions like anger, rage, guilt, shame etc suddenly without much control. Introverts can also suddenly be very expressive verbally or physically.
