Sometimes Engineer's Squares aren't

[routercnc]

Whilst trying to sand a wooden block square recently, continually chasing around each corner with an Engineer's square and sanding it back, I kept finding that the last of the 4 angles was not square :confused:

Puzzled, I started to go through all the options.
1. I was going mad
2. An uninvited black hole had entered our Universe and distorted Space Time such that Euclidean rules no longer applied
3. The square, was not.

As you can see in the attached photo it was option 3 - checked against several known good squares. Made a nice sound as it hit the bin. Lesson from this is be careful buying cheap tools from market stalls . . .