There are an awful lot of different connectors, depending on which beer you're talking about, and each requires a slightly different technique when coupling/uncoupling the keg. Some are 'lift-and-slide' (Guinness), some are 'lift-and-twist' with a three-point connection (Tennents), and quite a few are 'lift-and-twist' with a modified bayonet connection like this.
There are a few things which can go wrong with these.
The simplest is what's called a stuck spear. That's when the valve on the pipe which goes down into the beer inside the keg fails to close when you remove the connector. The result is a beer shampoo, or at worst, bath, as the entire pressurised keg empties itself all over you, the ceiling, and the rest of the cellar. In our experience, Guinness kegs are worst for this.
Slightly more dangerous is when one of the bayonet-style connectors gets stuck, and instead of the connector coming off, the spear starts to unscrew. If you don't notice, there is the possibility of a beer shampoo. Far worse, however, is if the spear actually comes uncoupled and flies out of the keg under pressure. That can do you serious mischief.
Today, the Belhaven Best connector got stuck. It's happened before, and generally all you need to do is tighten the connector again, give it an extra hard twist to lock the spear in place, and take the connector off.
Not this morning. Ominous hissing noises ensued, and after three goes, I phoned the Technical Services people at Belhaven. This is not something I wish to have to deal with.
So, today the beer line cleaning will be one line short. Not great, as the only way to keep your beer in top condition is to sterilise the lines weekly - and doing that twice over will not be fun.