Anything that looked like a good idea at the time, but has become a quagmire from which you cannot retreat gracefully.
A relationship that is unpleasant but can't be dropped.
An unsolvable problem or bad situation you should have backed out of a long time ago.
I thought my boss would appreciate this new project, but it's Viet Nam.
"I thought you were getting along with your brother now?"
That party was Viet Nam.