Mine did that once when I was in the process of setting them up.  Power cycling fixed the problem and I haven't had it happen again.  They can be a pain getting to run some times.  Any time I stop mining on them I have to power cycle and reconnect the fans or I just get connection errors.

Chad

Yep, any time a unit just spits HW errors upon starting, I disconnect its power cable and let it sit for a couple minutes.  When I reconnect it (and dis/reconnect fan), it hashes just fine.  This can be done without restarting cgminer, so if you have other units, they can hash away during the waiting period.  It probably happens 5-10% of the time, but only at the outset of a mining session; long mining uptime is key to minimizing this extra effort.