Every time an item is repaired through the use of the Mending enchantment, added in 15w42a, it "bounces" in your hand - disappears and appears again, as if it was being replaced or picked up from the ground. This makes it impossible to use tools at the same time they're being repaired, since it takes the same usual cooldown for each tool to reappear.
Steps to Reproduce:
1- Get any item with the Mending enchantment.
2- Put it in your main hand or off hand.
3- Collect experience. The item will "bounce", as if it was being replaced.