I finished painting my bedroom. There was an area behind the bookcases I didn't paint
because it was hard to reach. Now I want the bookcases in another room.
I had, however, put away the leftover paint. That was in 1988.
Says so on the label of the paint can, still barely legible.
Needless to say, it resembled a brick when I opened the can. Topping up the paint with water, heating and stirring occasionally it gradually reliquified. When it was nearly paint again, I stirred it well with an electric mixer.
Went on fine.
= new reply since forum marked as read
I just put it on an electric stove burner on low for an afternoon.
And I just used a hand electric mixer.
Oil paint would be dangerous.
I was a little tiny bit afraid the paint wouldn't dry, but today it's undetectable from the rest of the wall. It's all the same age, anyway.
over a spot that was painted a long time ago. I thought that paint changes over time on the wall, and trying to do a fix on it would not match. Goes to show----don't listen to everything you hear.
Edit: and I knew it was for latex only---or the water wouldn't work. 
told the painter to skip behind the breakfront. No one would ever see it. I had guys from my job come in a 5am and we moved it. 
He was shocked.
I've not been so lucky with old paint. 
Repaired a hole from an a/c unit ended up getting a gallon because the old stuff dried like a rock.
