Going through a Nether portal causes the quantum suit to go into some kind of hyper-mode. Sprint+jump will catapult you at ludicrous speed, and hitting anything in that sate will send you into the stratosphere (causing disconnection for hacking or flying). Even without sprinting, jumping while moving seems to be very twitchy. Just sprinting, or just jumping (without holding a movement key) seem to be ok.
Disconnecting/reconnecting will fix it. It appears to be the act of using the portal that messes things up, not simply being in the Nether, as the problem exists in either world, and is solved in either world by relogging.
Possibly separate issue(s):
In the nether, q-boots will jump high, but don't work to prevent fall damage.
In the nether with the q-helmet, my hunger bar drops to about 40%, then fills back up every few seconds while sprinting.
I've only tested this in SMP so far, and has persisted from 1.23-1.337b...