The square knot is somewhat over-rated; for joining two
lines of similar size together, you might be better off with
a sheet bend. I also like the Carrick bend, but that's
a much more obscure knot.

The square knot is the same knot as the reef knot.
You can think of a square knot as being made up of two
consecutive overhand knots, executed as mirror images.
If you mistie a square knot (using identical instead of
mirror-image overhand knots), you might end up with
a granny knot. Finally, even if you tie it correctly,
make sure you tighten both ropes equally; if you
over-tighten one, the other rope will pop out and into
a larks head hitch.