And it takes five times longer than you thought?
And you end up buying loads of miscellaneous parts you hadn't thought about?
And half the stuff you planned to use doesn't work?
And you spend way more than you planned on new stuff?
And you become inexplicably obsessed with colour coordination?
And you spend so much time finishing the fiddly bits at the end that you're sick of the sight of the thing before it's turned a wheel?
It's still ruddy ace when you finish it.