I've experienced this obnoxious bug with the pie slices where if I want to have 1, I need to buy 2 but still only end up with 1.
In more detail, if I buy 1 pie slice it doesn't show up in my inventory, but if I buy 2 they both show up except if I use 1 of them, they are both "used" as if I only had the one. If I got 2 energy from it, it wouldn't be much of an issue but I don't, so it is.