Search 'defect' and I bet you'll find plenty of stories

The police can pretty much check for defects at any of the times you have described above.

Usually they will write out a ticket describing what's defective, and the corrective action (e.g. get a blue slip inspection at a AUVIS, or an RTA inspection).

Common ones I can think of: too loud exhausts, vehicle height too low (suspension), incorrect wheel sizing, incorrect tyre load rating, GT wings, carbon bonnets, etc.