The word 'obviously' is key here!
Our eye finds it hard to spot that kind of small difference in a regular pattern, especially when it's broken up along a line.
There are other similar cut-and-rearrange paradoxes that break the missing square into a long thin diamond shape as well, but this one is particularly neat.
(
flatfrog Staring vacantly into space, Tue 1 Oct 2013, 12:24,
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