Recently, I found out that you can spellsteal more than one haste buff from Amana'shi Flame Casters in Zul'aman. The buff increases your haste by +300% (and also your speed, which sadly does not stack). Before I discovered this, I used the Cookie fight in Deadmines, but even with haste gear, Icy Veins and Time Warp, I only got to ~6300% haste where everything was more or less still working as expected. So naturally, I wanted to see what happens when you have more than 10000% haste.
There are 11 Flame Casters in the whole instance, here's what happens when you steall all their buffs:
First thing that broke was mana regen. When I got 4th buff, my mana regen dropped from 1.1M mana/5sec to 14.4k (which is the base mana regen for mage with zero haste). This was actually surprisingly problematic, because for Frost/Fire Spellsteal costs 33.6k mana so you'd run out of mana before you steal all the buffs. I actually wanted Frost for the extra +30% haste from Icy Veins, but had to go Arcane for cheaper Spellsteal.
Mana regen also broke in another funny way. Evocation started consuming my mana instead of restoring it:
Channeled spells in general were acting weirdly. Arcane Missiles channel time was so fast that it actually didn't launch any missiles. And it wasn't just a missing visual effect, the whole spell did zero damage.
Testing melee damage would be harder, as the mobs have very low hp. Luckily for me, if I unequip my weapon, my melee damage drops down to around ~11. Despite this, I was doing roughly 10k DPS. Sadly not the expected 4M+ dps, so melee swing timer cannot be reduced below certain point (I assume 1000 swings per second).
And perhaps the most hilarious result was that my spells became instant. Sad that you cannot use Presence of Mind for teleports and portals? Worry not! They behaved like instants in every way, which also means you could cast them while moving.
If you want to try this yourself, you'll need to go Arcane or (if you don't want the full 11 buffs) just bring a mana potion. There are 10 Flame Casters between Nalorakk and Jan'alai and one extra Flame Caster in the trash packs before Hex Lord Malacrass.
You also want to make sure the Flame Casters buff themselves at roughly the same time. The best way to do this is by pulling two at the start, polymorphing one of them. When the other one casts the buff, you break the polymorph (just poke them with your fists) and he should cast the buff in few seconds. Then just continue pulling them one by one, polymorphing the newcomer and syncing the buffs in the same way.