I suspect that I blew the circuit when I accidentally reversed the voltage. I didn't figure it would matter so much since it's "only 5V", but when reading the datasheet it appears that the circuit is sensitive to wrong polarity. It says on page 4 on the datasheet:
"Take a measure against Vcc voltage rise due to reverse connection of power supply and back electromotive force."
and the PCB has no protection circuitry against wrong polarity.
But maybe the coil has shorted. It had no problems cranking the fan blades when I fed 5V directly to the coil though so it appears to be normal.
There is MathJax support, yay!
\({{\left( \sum\limits_{n=1}^{\infty }{\left| {{x}_{n}}+{{y}_{n}} \right|} \right)}^{{1}/{p}\;}}\le {{\left( \sum\limits_{n=1}^{\infty }{{{\left| {{x}_{n}} \right|}^{p}}} \right)}^{{1}/{p}\;}}+{{\left( \sum\limits_{n=1}^{\infty }{{{\left| {{y}_{n}} \right|}^{p}}} \right)}^{{1}/{p}\;}}\)