This has only happened on creative mode (I've only seen it there) but it might be on survival

I got out an ocelot, It looked at me and I gave him raw fish expecting a cat out of it,
But all I got was some hearts and a nice ocelot.

Steps to Reproduce:
1. Spawn ocelot, In a closed room
2. It look at you
3. you give it raw fish
4. Instead of turning into a cat it just has tons of hearts over its head when you feed it.