Lack of people
When you turn up in your short skirt, to find the host, her nerdy friend and nobody else.... you remember you forgot you have an exam the next day and must go home to do some emergency revision, like NOW!

When somebody breaks the glass door
Otherwise you'll be roped into cleaning it up, which is never good when one is slightly inebriated.

When you break the glass door
Quick! Before they realise it was you!