Sent out on GFT to pre-flight a C152 that had had its 50 hour check about a week earlier, but had flown about 20 dual hours in the meantime. Checked the prop and got down low to check the air intake (below the landing light) and found out it had been installed upside down and deformed itself and the cowling. My GFT was postponed until another aircraft was available, but got me a shed load of brownie points with the examiner!
Not so much a pre-flight check, but pre-flight nevertheless, on my the final leg of my qualifying cross country when taking off on Shoreham's bouncy grass I discovered the spring holding the seat lock in place had failed. Hit a bump, back goes seat as does the control column and throttle! Luckily I had just enough flying speed and arms like a gorilla so I was about able to sort myself out. I hate to think what may have happened if I was slower/less monkeylike.
Gareth.