I recently played a game of URF as Teemo. I went back to the fountain to buy items and bought an item before Teemo went into stealth mode. As I went to undo the purchase Teemo entered stealth mode and the undo button grayed out.
I don't know if this specifically happens on URF or every map/game type.
Map: Summoners Rift Game Type: URF Champion: Teemo Skin: Panda Teemo
Yeah the refund button is bugged.
If you buy an item and then refund it while healing, it refunds your HP and Mana.
I actually saw something about this recently, maybe on Surrender@20. Because of programming intricacies (like, how much HP should you have if you refund a Warmog's—I know it sounds stupid, but I'm not a dev either), when you buy something the system notes your current/max HP, mana, etc. If you refund something (@Daedalus871 this answers your bit) it reverts to those values.
The moment you change state (e.g. stealth, Karthus ults, etc), the system disables undo. The reasoning behind it is, what if e.g. you recall with only a couple hundred HP, see Karthus ulting, buy a Warmog's or a Zhonya's, survive, then refund it. Clearly unfair, right? So they solve it by disabling undo after you do anything.
That's not to say it's perfect; it would seem that for Teemo they probably should work around that. But hopefully you understand the "why" a bit better.
Sometimes as Teemo my homeguards don't work :(
Shrooms doing damage counts as combat. doing damage means you can no longer undo.
