How to reproduce:
1. /effect player 21 30 4
2. Heal up 10 extra hearts
3. Wait for health boost effect to end
4. Try to take damage
For some time you can't take damage, I think this happened because those 10 extra hearts weren't removed correctly, thus after taking some more damage health bar finaly starts to go down (health overflow is hard to observe, as scoreboard is bugged too)