Caps on Mana
Some more quirkiness with mana, the total amount of +mana you can have on a single piece of gear is 1897, no more.
The total cap on your mana pool is 65535k. Anything above that loops around and brings your mana down to 0 on the client, so you cannot display any values above that. This results in some issues like it thinking you do not have the mana to cast a spell, for example. Also, 50k mana is the cap for Mana Sweep, you cannot get further damage above a mana pool of 50k with that skill.
I played Arcane Torrent Druid this season and I wanted to add a little information about how mana values above 2^16 -1 are handled.
The displayed value in the bulb does in fact roll over and shows the actual current mana - 2^16 and therefor the displayed damages of Mana Sweep and AT are bugged, too. The current mana is set to actual current mana - 2^16 and the lost mana is set to 2^16 (actually 50k cause it is the cap).
The dealt damage on the other hand is calculated correctly.
That means if you can overgo the value of 2^16 by a decent amount - so that casting spells is actually possible - you can raise your AT Damage to pretty high values.