Throughout the Mists expansion, an unaddressed quirk had allowed rogues to cause trouble while stealthed for much longer than was intended.
Resolved an issue where Vanish modified by Glyph of Vanish was incorrectly causing the Subterfuge effect to trigger twice. Vanish will now cancel its effect when the duration of Subterfuge expires.
In other words, what was happening here was that a rogue would cast Vanish, which would activate Subterfuge. Normally, both the “improved stealth mode” granted by Vanish and the Subterfuge buff last for three seconds, meaning they drop off at the same time. But the Glyph of Vanish makes that “improved stealth” last a total of five seconds. So it caused Subterfuge to actually proc twice — once when the rogue first hit Vanish, and then again when the five seconds wore off, extending their stealth by an additional three seconds.
(Correct me if I’m wrong, but if a rogue used Preparation after this, they could theoretically do everything all over again with an additional Vanish, actually giving them 16 seconds of stealth while still actively wreaking havoc.)
From what little I know about this bug, the value of exploiting it was not tremendous, particularly in PvP. Players had known about it for quite some time, but because it requires PvP rogues to burn a hugely valuable Vanish for somewhat dubious benefit, its relevance in competitive environments (specifically arena) was pretty limited.
It was probably utilized much more frequently among Subtlety raiders who were very serious about optimizing their DPS rotation, since its use during raid fights would have yielded more Ambushes (and, I’m assuming without actually pondering numbers here, better Find Weakness uptime).
But! Not anymore. :)
But but! There’s at least one upside: