Select two buff spells (or anything really as long as it can be cast in town for easy testing), give one the Quicken (always off) overwrite, leave the second one with the default setting.
-Turn quicken on. Try casting the first spell (one that has an active overwrite) - sometimes it will cast quickened, sometimes not, even though it should never be quickened. Turning quicken off and back on sometimes solves the issue, but sometimes it doesn't. Also, the glitch seems to carry on to other spells with no active overwrites - after turning quicken on and casting a spell with an active overwrite (quicken always off) a few times, there's a chance that a spell with no overwrites at all will still ignore the quicken.
-For example, I'd put Quicken always off on haste, no overwrites on prot elements. I turn on quicken, cast haste 10 times, it got quickened the first time, not quickened the next 9 times. Then I cast protection elements (with no overwrites), it gets quickened. But then, on the next cast, the haste (still with Quicken off overwrite), gets quickened. After that haste, I cast another protection elements (no overwrites) and even though quicken is still on, it doesn't get quickened.
It *seems* like the overwrite of a spell cast affects the next spell cast, whether that spell itself has an overwrite or not. However, that doesn't explain why that first haste, cast right after quicken was turned on, got quickened.
I've tried reproducing the same thing with the overwrite for extend, but out of 20-30 tries, that one has worked exactly as it should every single time. Don't really have a reliable way to test others right now.