(a.k.a. Musical Theater Tourette's Syndrome)
The act of randomly breaking into song (or song and dance) during conversation; most commonly seen in musical theatre actors.
Often, actors are falsely diagnosed with MTTS because they are really only: a) showing off their knowledge of musical theatre, b) secretly auditioning for any directors that may be in the room, c) drunk, d) just generally annoying and are living up to their reputation.
However, many scientists have concluded that; for non-musical theatre people (a.k.a. normal people), there is no such thing as a falsely diagnosed actor and hence all musical theatre actors are inflicted with MTTS, regardless of their intentions.
The most common side effect of MTTS is being asked to leave a restaurant.
Other reported, but unconfirmed, side effects are: a.) being told to shut up b.) being the target of food thrown from across the room/restaurant c.) having people roll their eyes at them d.) never being invited to parties e.) being expelled from the "cool people table" f.) never being cast in another show.
In extreme cases it has been reported that actors inflicted with MTTS have actually been "de-cast" from a show, even during the run of the show.