Sometimes when fermenting in a glass carboy, you run across the problem of the headspace filling up with a lot of foam.  That foam can work its way up into the airlock and jam it up.  If this happens and the pressure is great enough, you will either end up with a geyser of beer shooting out the top as the airlock shoots out or, even worse, a shattered carboy, depending on how hard you crammed the airlock in.

Here some ways around this problem:

1. A really large (1 inch) diameter sanitized blow-off tube.  Cram it in the neck of your carboy and put the other end in water.  This is by far the best way.  It is borderline impossible to clog and will still keep a sealed environment.
2. Some airlocks have a little grid structure at the bottom which can clog.  Cut off the very bottom of your airlock (the part where the four little plastic arms come together) to give yourself a lower probability of that part clogging.
3. Remove the "cap/disk" of the airlock.  This is the other area that gets clogged easily.  If you have a three-piece airlock, the inner hood may pop out in a really vigorous fermentation.  As long as it keeps flowing out, this generally is not a problem.  I've been able to create a "cage" for the hood using a rubber band crossed over the top to keep the hood in place after I've removed the "cap."