In a local multiplayer game,I had stone in the first slot,but in the 3rd slot were rails,which I was using.When i would place the rails,it would place stone.

Me and my friends were playing on a local server game,everyone was on a 3rd generation Ipads.One of my friends said he was trying to place stone,but got rails.He then experimenting with different blocks,putting some in the first slot.He also tried to farm,had a grass block in the first slot,you know where this is going.

Steps to reproduce
1.Play with people while connected to a public area network,preferably with alot of other people connected.
2.put a block in the first slot of the hotbar,and use another block in a different slot.