Both my big need-hospital-treatment crashes happened on easy trails, the type of stuff I've ridden countless hundreds of times before. The first time (in Verbier) was a gentle singletrack descent – OK, I hadn't seen it before but compared to some of the trails we'd ridden that week it was dead simple.
The second one (in the Lake District a couple of months ago) spawned a long thread on here but the cause of the crash was a tiny jump/drop-off on grass, not exactly a Josh Bender style cliff gap jump. I've still got no idea how I screwed up either bit of trail but both required helicopter evacuation. At least when I do f*** it up I do it properly!
I think on perceived easy trails or ones you know well there's the tendency to treat it a bit too casually sometimes.