What I was doing:
Running around a plains biome doing my stuff. Needed to fill hunger bar

What I expected:
To eat the item and continue walking.

What actually happened:
Consumed the item, then while walking and jumping (not holding any mouse buttons) another food item gets consumed without any animation or slowdown

How to reproduce this:
0. Have more than 1 food item in the same slot (e.g. 2 apples)
1. Walk while eating (hold W and right mouse)
2. If you time it right, right when you finish eating in that split second, jump and then use CTRL to run while letting go of right mouse just as the animation finishes.
3. If followed procedure successfully, in about 2 seconds, another item will be consumed without any animation nor slowdown effect.

PS:
This has actually been an issue since Beta 1.8 I believe.