The teachers in my school were far too ‘savvy’ to allow anything to interrupt a caning. In any case the ceilings were very high (the orginal building dated from the 19th century), so there was never any chance of the cane coming into contact with it.
I can never recall an occasion where a classroom caning was abondoned, and I never heard of it in the headmaster’s office either – unfortunately!
A bad caning generally involved a victim being unable to take it in the normal Stoical way.