I'm not sure if this is a problem for other DGs or not, or if this is a product of lag, but I found that if you play regulus and use mark on a distant enemy so that reg persues, you won't be able to issue a snipe command on that target without first issuing a move command - it doesn't seem to abort the attempt at marking until you actually catch up to and mark the enemy.
I'd expect that the snipe command to abort the mark command.