Why do you sometimes manage to *uck up?
I planted myself tonight on the Follow the Dog at Cannock Chase - my very local ride... on a bit that has never been an issue before... bummer.
I knocked a big hole in my knee on a trail I built, on a branch that I know for a fact wasn't there and doesn't exist 
cos the mind wanders... That's my excuse anyway!
More complacent or maybe riding a bit more on the edge because you know it so well?
what chvck said, hurt myself last year cos I was caning it down my fave local trail, on a fast corner I tried using a tiny little gulley as a berm same as I had all summer - only it had rained hard the night before and turned it to sludge. Sludge doesn't make good berm material. Ouch.
You know it so well you're riding faster, on 'autopilot' kind of, therefore any 'out of place' rock, branch, even a little unexpected mud can have you off....whereas on a trail you dont know that well, you're reading the terrain more as you ride
thats my thoughts anyway
Years ago I had a huge crash on a trail at Trellech, out like a light for about 5 minutes, helmet smashed, front wheel wrecked must of done it about 100 times before, still dont know what happened. Famiiarity breeds contempt maybe?
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.
I just don't concentrate on trails I know well so it all goes fine until I come across something that has changed and my memory no longer keeps me upright!
Not mtb but I ride the same MX track every wednesday I crash on it about once a week as well! I put it down to complacency. On a side note do you really know it well, or are you riding the same line every time. I was marshalling on the track the other day and everyone rode the same line through a corner I imagine I ride that line as well, however, one gent was keeping tight to the inside, avoiding some accelleration bumps that everyone was hitting and he was on the throttle for longer. Such a simple thing was giving him time over everyone else. Moral being might be worth looking a bit more closely at your local trail and trying some new lines if nothing else it will make it more interesting (and you will be concentraiting more so wont crash!)
Iain