Same problem, more or less, with any regulator. Depressurize the system underwater and you run the risk of water getting past the 2nd stage valve and entering the hose, gravity does the rest. It's a small risk but we do see it on the repair bench with some regularity. The risk is mitigated (but not eliminated) when you re-pressurize the system and run gas through it, forcing any water that got into the hose back out through the 2nd stage. This is more of an issue with stage/deco rigs than back gas, which tend to be left valved off for extended periods with the hoses in a nearly horizontal position and can have the purge button bumped repeatedly.
Minimize the problem potential: When you're doing a valve drill, breathe the regulator down but don't overdo it - when the gas is gone, you're done. Don't hit the purge button when the system isn't pressurized. For bottles you're slinging, consider feathering the valve open just a smidgen to allow the system to re-pressurize if you accidentally bump the purge. Blow the system after you re-pressurize it. Rinse well (really well with lots of running water) after every dive. Overhaul your regulators regularly - like most problems, early detection equates with better outcomes. Pretty much the same stuff you've been hearing since OW.
