Keys can be checked out to provide access to special facilities. For example, the Computer Room (StJohn 606A), the Herbarium (StJohn 401) or the Microtechnique Darkroom (StJohn 417).
If you lock yourself out of your office, or the building, you should call Campus Security (x6911-- there is a pay phone near Hamilton Library). They will come let you in; be prepared to show identification.
Faculty members have master keys and can sometimes help in an emergency.