If you have a status effect and, in the inventory, hover an item in one of the last vertical rows of the inventory, its tooltip will be shown to the left of the item rather than the right, since it wouldn't fit. However, if the name is long enough to reach the status effect indicators, they will be shown on top of the tooltip, making it not readable.
Steps to reproduce:
1- Give yourself a few status effects.
2- Open the inventory.
3- Hover an item with a long name or tooltip. Enabling advanced tooltips (F3 + H) might help.
4- The status indicator is shown on top of the tooltip.
I'm pretty sure I've seen a ticket treating about this bug a long time ago in the bug tracker, and it was even fixed at some point, but, after noticing it was still present, I couldn't find it by searching for effect, inventory, tooltip and gui. But maybe I'm wrong and it was never reported or fixed.
Found it! MC-12503