Here's a few of the defects I've noticed over the years that would have required the bus to be taken out of service:
I travelled on a Stagecoach 219 bus between Ashton under Lyne and Fairfield Street, Manchester. Every time it stopped at a bus stop, the driver got out of the cab to close the doors because they failed to shut properly when the button was pressed. At Fairfield Street I overheard the driver saying he needed to take it to depot.
A First Leeds Wright single decker had a window where water was trapped like a goldfish bowl between both laminated glass panes and it sloshed about depending on the vehicle's acceleration and braking.
A Harrogate Bus Co Wright single decker on route 7 working from Leeds to Harrogate stopped on Coal Road, Whinmoor when the driver suddenly noticed no lights on dashboard. After phone call to depot he told the passengers on board, the speedometer lead was broken, and was told by the depot to proceed to Harrogate without a working speedo. With his passenger load, he then drove much more slowly and carefully.