I had a higher-end listing that we put on the market in 2005, and we got a contract pretty quickly quickly (not right away - maybe three months). The buyers asked if they could move in a week or so before the closing, because they have seven kids and they were staying at a local hotel which was costing an arm and a leg. Although we were all a bit skeptical and wary of letting them move in early, we decided to allow it, but we put in a rather punitive amount for the "holdover" period. In other words, if they didn't close by August 31st, 2005, they would be paying $300/day. We thought this would be sufficiently motivational to get them out or get it closed.
Boy, were we wrong! They paid this amount and then continued to pay for about a YEAR! Yes, they paid $9000/month for almost 12 months. They kept claiming that the Patriot Act held up their money crossing the Canadian border, or that there was some other snafu. They would get behind for a month or two, then pay a lump sum of $27,000 to the seller. She finally got tired of dealing with them and decided to evict them for non-payment. The sheriff's office had to forcibly evict them from the property.
Needless to say, this was the longest time I have ever had a home marked "pending", and it didn't close with that buyer. We got the property cleaned up and repainted, then we hired a professional home stager to come in and work her magic. The home was under contract again very quickly, and we closed not quite two years after we had originally listed. The seller told me that I had really "earned my money" on this one, which was gratifiying indeed.
Note to self and to others: If you can avoid it, DO NOT let buyers move in before the closing occurs!