I thought our cats were generally stupid because they could never do the jigsaws I assigned to them, solve a Rubiks cube or see 2cm in front of their faces (they are long-sighted).
But I have come to believe they are far smarter than me, particularly with scheming methods of sneaking around to quietly appear when they can get food. One used to push donut boxes off the side so that they would burst open, and would then run off with a donut. I have no idea why he wanted to eat donuts. He also tried to eat bread through the wrapper.
Or they fein illness. Our one cat had an operation on his leg so we put a little box as a stepping stone so that he could get onto the bed, and then onto the window sill. Turns out he didn't need it at all and could jump perfectly fine, but would still use the box all the time.
They're really clever, just not at jigsaws or sudoku.