So you have lined up the perfect shot only to have someone walk right in front of you at the perfect (or worst) moment. Having people in your way when shooting an event can be a constant annoyance for any photographer. You can get frustrated or you could do what any good photographer would do and deal with it.
Solution: Use that observer as a part of your framing. When you add an observer into the mix you are creating a photograph that tells a story. The photograph becomes more interesting. There is also an added context of the person who views the photograph feels like they are a part of it themselves.
So much of event photography is just dealing with the senarios you are presented with and thriving in them. You have to react in seconds (often less). After all you can't publish excuses.