If you use a command block to summon an xp orb in an enclosed space, if it is in your pickup range, it will look like you collected it but you didn't.
Steps to reproduce:
1. Dig a 3-block deep 1x1 hole.
2. Place a cmd block in the hole.
3. Set the command to "summon minecraft:xp_orb ~ ~1 ~"
4.Fill in the top block of the hole, leaving a block of air above the cmd block.
5. Power the cmd block.
6. You will be shown the pickup animation, but you will not pick it up.