Its not a major or exploitable bug, but when the earth traits Written in Stone (maintain signet passives when recharging) and Signet Mastery (20% faster signet recharge) are both used, the temporary passive signet effect that is added when the signet is used stays active after the signet has recharged. It stays active until the normal recharge time, by which point the signet is already giving its normal bonus, which doubles the intended effect for that last 20% of what the normal recharge time is. So, for example, Signet of Earth will give about 6 seconds of double its toughness bonus.