Re: Impact Assessment
As you say, it's a "Risk matrix" thing
Ranking the issues by taking the likelihood of something happening "multiplied" by severity of event if it happens.
As the British HSE says "It is suitable for many assessments but in particular to more complex situations. However, it does require expertise and experience to judge the likelihood of harm accurately. Getting this wrong could result in applying unnecessary control measures or failing to take important ones"
In this case, probability of bad thing happening 1/100 (estimate but sounds like it was based on wrong premise - that the work would be carried out absolutely best practice) times the severity of an incident (Whoa! Overtime forms out. Is my CV up to date?) = really serious risk.
Outcome - decide to reduce the likelihood of the event, or the severity of the event.