Just ask Cardinal outfielder Matt Holliday. He'll tell you.
Friday Holliday tried to make a play on a foul ball. At the time his team was already down by three and their were two players on so every out was critical--but a security guard failed to get out of the way in time.
In the end it all worked out. Holliday hit a single, double, and home run and the Cardinals won 7-6.
Rather than make the catch (which it looks like he could have) he ended up smashing in to the security guard instead.