After years of use and abuse, levels aren’t always level and squares aren’t always true, especially the cheaper models. Before you throw them away, let’s check out your tools to see if you can rely on the readings they’re providing you.
To test any level, put it on a flat surface and shim up one end until it reads level, then turn it 180 degrees and read it again. If it isn’t an identical reading, it’s not accurate, and it’s probably time to buy a new one.
The same goes for a square. Line it up on a straight edge, and draw a line with it. Then flip it over, and draw another line on top of the first one. If the lines run apart, your square isn’t square anymore.