Worst interpretation of a rule you’ve ever heard?

I’ll go first. I used to think you got two actions instead of an extra attack.

When I first started DMing, I would allow my players to take a third action. I started back in 3.5 when you could take a Full-round action/attack to make a second attack, but in my head, I rationalized this as forfeiting your movement to get a second action. So when 5e started, I thought you could do a full round action still, getting 2 attacks at level 1, and a third attack at level 5. So I used to allow my players to take an additional action at levels 1, 5, 11, and 20.

This is why I have always thought martial classes (especially the monk) were super powerful. Far and away outclassing even full spellcasters.

So what about you, what was the worst interpretation of a rule you’ve ever heard?