So let's say, for example, I have a diamond shovel in one hotbar slot, and a diamond pickaxe in another. Sometimes, presumably when switching items, the texture of the diamond shovel in hand is actually that of a diamond pickaxe. The texture and name inside the hotbar doesn't change, the item will still act as it should, and the item it is "mimicking" is completely normal. Destroying blocks with the item (decreasing its durability) does not fix it.
It is not a common bug, but still enough to notice from time to time.