This crazy story describes how a 300 lb Siberian Tiger was able to get out of its cage at the San Francisco Zoo. It killed at least one person and attacked two others before it was shot. Zoo officials have no idea how it got out, since the one door into the enclosure was locked. I guess if you want to do something badly enough, you will. This tiger had also previously attacked one of the zoo employees.
See Tiger Escapes S.F. Zoo Cage and Kills 1