Never overestimate the amount of grip semi-slick summer tyres will give on said hill.
Never underestimate the chances a rock will come into contact with your knee when you go arse over elbow on hill.
Never overestimate your pain threshhold when you get two inches of North Yorkshire's finest sandstone inserted into your knee joint.
Never underestimate your ability to carry on biking to get home when you've no mobile and no choice.
Never overestimate the amount of time it gets taken to get seen in casualty if you walk in actually bleeding (they don't like it on the floor).
Never underestimate just how painful 'irrigating' a wound can be - my, my it smarts.
Never overestimate your ability to walk afterwards.
Some tendon and nerve damage, but I'll live to bike another day. Worst injury I've had in 12 years of mountain biking, so pretty lucky really.