Accident waiting to happen

Hi,

I read this sentence 'The exam was an accident waiting to happen'

Is that some sort of an idiom - why didn't they say 'a disaster waiting to happen' for instance and what is meant by the idiom anyway- that they expect the exam's going to be hard?


Thank you in advance.

[rewboss]

They could have said "disaster" -- but either way, yes, it's an idiom.

An accident waiting to happen is a dangerous situation that is almost certain to end in an accident: for example, a banana skin on a footpath might be described as an accident waiting to happen.

In this context, I think the idiom doesn't quite fit, and "disaster" might make more sense; but it is obviously intended to mean that the exam was expected to be extremely hard, most likely because the students hadn't prepared for it properly.