Malzahar can't undo an item buy while malefic visions is damaging a unit.
This just happened in a Ranked game and, frustratingly, caused me to get a bit behind in my lane.
I was killed by Zed and immediately opened my shop, purchased a Catalyst, then realized I ALREADY had a Catalyst. Silly me. Went to undo, and the option was grayed out. Quickly checked and noticed that Malefic Visions was still damaging Zed. My death timer had upwards of 20 seconds left. Ended up having to sell the extra Catalyst and buy another item. At his point, I could undo purchases, but I was never able to undo the first purchase of the extra Catalyst.
Haven't had time to try replicating the issue.
Hope we can look into this, it was pretty problematic and caused the opposing laner to get some important items ahead of me which can make or break a lane. Especially in Ranked.