Because that's not a corner (anymore), a corner has a single anchor point.
It was a corner about 5 minutes before I used Rounded Corners. So there's no going back?
I'm sorry, I mean that It was a straight corner, then I rounded it to 75px and then went back to undo and it didn't allow me. How does it change from being a corner to round to no longer being a corner?
It could be a software glitch, it could be that you accidently moved/edited one of the anchor points tha appear when a corner is rounded.
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