It is estimated that around 15-20% (depending on country) of all accidents and around 10-15% of all fatal accidents are related to maintenance operations.
Maintenance, repair, tuning, adjustment is fourth on the list of top 10 working processes accounting for the highest number of fatal accidents over 2003-05 (EUROSTAT-ESAW). Accidents increasingly tend to happen not during normal operation, but rather during repair, maintenance, cleaning, adjusting, etc.
About 38% of the chemical accidents are caused by dangerous materials released during maintenance (Koehorst,1989).
This data is from OSHA ...
Ed.