Slingload tutorial -- problems w/ payload

In the mission "Slingload Tutorial", as soon as I hook on to the pile of crates, they bounce up into my chopper and register a crash! Is this a software bug, or is it something I'm doing wrong (altitude too low, too much airspeed, etc.)? If this is pilot error, how can I avoid it in the future? If it's a bug, what (if anything) can I do about it?

Never mind, I just discovered it was pilot error (not enough altitude). Though if it was for real, it wouldn't result in a crash, as I was still clearing the crates by 20 feet or so.