Scald due to common waste line
Two reactors in a laboratory. The outlets were each 12mm diameter. Unfortunately, rather than have individual pipes all the way to the drain, the outlets were joined a short distance from the reactor, so that only one common pipe (of a similar diameter) went to the drain. Following each experiment, the operating procedures called for the operator to fully open the reactor, after opening the outlet valve - this to ensure ensure that no pressure remained in the reactor while being opened. Unfortunately, while one reactor was being opened, the other reactor started discharging. This filled the other reactor with steam, which injured the operator.
The correct way should have been to have individual outlets to the drain without joining. If it was not possible to eliminate a common pipe, then that common section should be of a much increased diameter, to prevent presurization and backflow to the other reactor.