Answer to

old chestnut: where did the other square go?:

This is a bit of a trick question, but not so much so to make me relegate it to the Riddles category...

The answer is that the difference occurs along the diagonal.
The difference is too little to really draw it properly in an ASCII drawing. This web page:
http://www.mcs.surrey.ac.uk/Personal/R.Knott/Fibonacci/fibpuzzles2.html#jigsaw3
has a better version of the figures.

In any case, the diagonals of the figures are not perfect. Neither one is exactly a right triangle. The diagonals of the two smaller right triangles don't line up. In one figure, they actually lie slightly below the true diagonal; in the other they lie slightly above it. The difference is one unit square over the whole figure, the square which is present in one figure and not the other.