..........
Video showing the glitch (version 1.2.8)

https://youtu.be/ZdKmqzdfmK4

As shown by the video, repeatedly using an item while switching between that item and another item will occasionally cause a slot to 'skip' the neighboring slot. For example, if I am in slot 6 and a 'skip' (for lack of a better term) happens, then I will switch to slot 4 instead of slot 5. As far as I know, this will not happen unless the slot being switched from holds an item that can be used (for example, a block, ladder, or fishing rod, and not an iron ingot, diamond, or ink sac).

This has actually been around for a very long time, so I apologize if I'm just reporting a game mechanic.

How to Reproduce :
1. Hold an item that can be used in slot B
2. Use that item, quickly switch to slot A, switch back to slot B, and repeat

If that was too confusing, just do what I did in the video. It shouldn't take more than three minutes to reproduce the glitch.