I played Jayce last night... a lot with a pre-made group of mine, I absolutely love him, but that's not my point.
(Can check my match history if you want)
Several times durring my gameplay bot, Jayce almost locked up, Wasn't able to use my Q in Crossbow form, Wasn't able to auto attack either. It happened during handful of fights bottom, which was extremely frustrating. It happened after a semi-long spell chain something like (Starting in crossbow form) : E>Q>R>Q>E>W That's the combo I use basically all the time bot for fights/harass.
I think its the combination of using spells to quickly and the champion locking up because of that, I was still able to move, type to my teammates, see the map and the minions work correctly. It wasn't lag or my computer. I just couldn't auto attack and use some of my spells. The way I broke out of it though, was switching to hammer form, and Q'ing a minion that fixed it every time.
Its quite frustrating though as all these happened during fights, which I obviously lost because of it.
Edit: Also this happened several times with Jayce, which means I was able to repeat the glitch. I don't know exactly how, but fast spell chaining is my guess with him.