When you take damage, the hearts you lost are supposed to flash white 3 times. However, If you take damage again, while the health bar is still flashing, it will reset the flash animation, but with too many hearts flashing. Instead of there being 2 overlapping animations, like there should be, there will be combined into one.

For example, if you take 2 hearts damage, then after 1 flash, you take 1 heart damage, what should happen is: 3 hearts will flash twice, then 1 heart will flash once. What really happens is: 3 hearts flash 3 times.

To replicate, stand on a cactus. As you take damage, the amount of hearts that flash will keep increasing, and will always be your starting health minus your current health.