Ok, so you know how you guys offer double XP in two different ways? One being an NPC to talk to which usually gives you 4 hours of game time worth of double XP (Time doesn't run logged out) and also a world-wide double XP that's always on like we just had for easter?

Well, if you had double XP active prior from a NPC it can be stacked with worldwide double XP to give you 4X. I stumbled upon this by accident; my non-subbed, PK mode account with a skull was getting 68X combat XP instead of 34X because he still had roughly 30 minutes of DXP left from many months ago that I never used, and obviously never really logged into the account since.

No idea if this can be stacked in any way with double XP eggs but that should be looked into as well. If this were to be abused someone could be pulling 16X XP for non-combat skills which would be pretty game-breaking, even if it was only possible for 4-6 hours max.

In summary... basically, activate a 4 to 6 hour double XP from the NPC (Usually Devil XP, I guess he's called) log out of that account and never use it until there's a worldwide always-on double XP, and there you go. 4X!