Guild Wars 2

Signet of Vampirism shows 25 siphons available when it is used on an opponent, but when I popped my wells/deathly swarm + dagger auto attacks it only siphoned 4 or 5 times when I clearly hit the sparring dummy way more times than that.

http://wiki.guildwars2.com/wiki/Signet_of_vampirism

wiki

Each attacker has a 1 second internal cooldown on this effect.

Most probably not a bug, but should be added to the description.