Space in the hotbar will randomly not allow items to be placed in it

When playing in survival, both singleplayer & multiplayer, (with/out mods & resource packs), there will be a time where a space in the hotbar will not allow items to be placed in it. Logging out or restarting the game doesn't fix it. The only way I have found to remove that locked space is to manually reset the inventory by dying or clearing the whole inventory by a command. It's quite frustrating to play with.