Eating boiled, cooked, baked or mashed potatoes that have been left out overnight is not considered safe.
When the potatoes are left out overnight at room temperature bacteria can grow on the potatoes and could potentially make you sick if you eat them.
However although I do not recommend eating any potatoes that were left out overnight I have personally done it a few times before and never got sick.
Now I could've got sick if there have been any bacteria on the potatoes but I guess there wasn't and I was okay.
The potatoes still tasted good but I did reheat them so maybe the bacteria was killed when I reheated the potatoes.
But I no longer do it myself and if I leave any cooked potatoes out overnight I throw it out to avoid risk of potentially getting sick.
If you need to keep leftover cooked potatoes it's best to refrigerate or freeze the leftover potatoes.