I'm loving this thread, particularly all the swine flu ones. But it's worth remembering that puns are one of the highest forms of humour when implemented correctly. The problem with bad puns (which I still love) are that they're just said with little to no context.
I'll take Airplane as an example. Some reporters are in the flight tower and have just finished asked some stressful questions to the staff. Wrapping up, one says "C'mon boys, let's take some pictures." They then proceed to casually remove all the paintings hanging on the walls and then leave. Brilliance.
Of course after seeing Police Squad the orginal series, the one that's really in my head isn't a pun, but a very dry, dead pan line (my personal favourite type of humour). The police, having just told a lonely woman that her husband is dead, are watching her cry. To console her, one police officer says, "I'm sorry Mrs. Johnson. We would have come earlier but your husband wasn't dead then."