Intermediate Admixing "tool-tip" states that only Alchemy I is required to learn Intermediate Admixing. The linked image shows that it clearly isn't true.
I've had this happen to me as well, but with a different skill - Transcendental Meditation. Not sure what's causing it or how to reproduce it, but here's a screenshot, if it helps.
If you notice, I'm actually learning the skill that it says I'm not qualified for, even though I do meet the requirements. Thus this bug isn't quite so severe: the requirements do function correctly and you can still learn skills when you meet the requirements, it's just a mistake in the tooltip.
There are two other threads, links available here and here, that discuss similar and possibly related issues. They have reproduction steps and further relevant info that might be useful.
Kev, I think this may be the same nasty gremlin you've been hunting down.
Doc, I use Chrome and ran into this mid-test last time and kev had me check to see if the issue persisted in Firefox. It didn't. Once I loaded in Firefox it cleared the issue. I logged back into Glitch on Chrome and the problems cleared.
Strange. My screenshot up there was taken in Firefox, but it's happened to me in both Firefox 4 and Opera 10. I don't think it's browser-dependant, but it might be one of those problems that appears and disappears due to some weird fluke or another.