I would say mostly I have had pretty good luck with my transactions but there have been many bad ones. It's too difficult to pick the "most" difficult but here are some that come to mind:
Two days before COE the water heater kind of "blew up" flooding a portion of the garage and kitchen (vacant house)
About a week before COE someone moved into the listing (vacant house) and claimed they were leasing the house. It wasn't the buyer who was in escrow -- it was someone who said that "Jose from craigslist gave them the keys". (this exact same thing has happened to me about 4-5 times)
During a 45 day escrow someone(s) broke into a gorgeous ranch (vacant house on 5 acres) and took the copper sink, all light fixtures (interior/exterior), all appliances (stainless), several of the interior doors and a wood burning stove. This occurred over a 2-3 weeks period until the buyer had their ranch hand start camping out at the property.
Again, vacant house - it wasn't in escrow but as a deal was being negotiated someone broke in and took ALL of the electric wiring -- and I mean they literally ripped the wiring right out of the drywall. They took the actual electric meter as well.