Standing room . In some theaters, you can buy a cheaper ticket that entitles you to stand in the back, behind the last row of seats. If you're hardy, this can be a great way to see operas. Occasionally, when the performing group is small and the audience is big, extra chairs are set up on the stage, surrounding the musicians. If you like to watch performers up close, this can be a great place to sit.
Some theaters have a few seats with only a partial view of the stage.
Maybe there's a handrail or a pillar in the way, or maybe the architect was a nut case. These obstructed seats are sometimes extra cheap. At almost every concert, there are people waiting out front with extra tickets to unload, and other people looking for tickets. More than once while I waited in long, discouraging ticket lines, a total stranger has approached me to offer a ticket. Sometimes the stranger won' t take any money, but just wants to see the ticket used.