I've been experimenting with some of the new blocks of the Nether Update, trying to make something cool out of them. For that reason, I've saved up some hotbars filled with different blocks and I started building. During my building session, if I wasn't fond of a particular placement of blocks I use the pick-block function (Middle Click) to get that block in my hand and fix that one part of the build. Once I'm done building, usually I'll open my "Survival Inventory" in the creative menu, Shift + Click that red X button and clear out my inventory.

Here is the thing, when I've done that after I finished building, some of the blocks in my inventory were not getting removed. At first, I didn't know why exactly this was happening, but as I tested this a bit more, I found the issue.

Basically, problems started ocurrring when I switched hotbars multiple times while I was pickblocking between them, then later delete my inventory. Some of the blocks were not able to get deleted, unless I put them on my hotbar or throw them away. When I switched hotbars only once and pick-block here and there and then delete my inventory, things worked fine.