Some easy starters in no particular order (note all are much better if you can perform in the dark using only a headtorch, or better yet, one of the free mini torches that fit on your keyring. Do I have to say don't test ride? Thought not) -
1) Don't fit a different cassette
2) Don't decide to 'just toe in the cantis to stop squealing' particularly if they work just fine as they are
3) Don't fit new v brake pads either
4) Don't fit a new stem and not do up the pinch bolts
5) Decide to do away with the spacers above the stem and cut the steerer to correct length. Using a blunt junior hacksaw.
6) Fit tubeless tyres for the first time
More?
1) Don't fit a different cassette
2) Don't decide to 'just toe in the cantis to stop squealing' particularly if they work just fine as they are
3) Don't fit new v brake pads either
4) Don't fit a new stem and not do up the pinch bolts
5) Decide to do away with the spacers above the stem and cut the steerer to correct length. Using a blunt junior hacksaw.
6) Fit tubeless tyres for the first time
More?