One annoying/effective guard in BJJ is the De La Riva guard (named after professor De La Riva). It is a form of open guard that creates a very tangled "weave" of the legs, and provides a large number of sweeping/back taking options.
Because of the entangled nature of this position, it is more difficult to pass (especially in the gi) than a lot of other guards.
Although the grips vary depending on who you watch use the guard, some principles always remain the same.
i.e. The person using DLR guard always wants to be at an angle to the person on top, and controls distance using BOTH feet.
Here is an amazing video featuring Shawn Williams showing a very methodical way to pass this guard.