I was playing karthus and then had a fully stacked tear, bought a blasting wand but didn't have enough gold for AA yet, then I went out and fought, died, and them before karthus passive ended I was looking at shop and spamming buy on AA. Bought AA after I actually died (passive ending) and then I notice my AA has 0/750 mana. Haven't had time to try replicating the bug yet but this is what I remember so far.

oh just did a search, looks like other people are having the same problem and with karthus