Lets look at it calmly.
Putting Hyd oil in an engine will not cause immediate damage and indeed much will be burnt away and diluted back to original strength on the next oil fill. Should an engine suffer ill effects, it will be due to repeated contamination and only then will bearing wear deteriorate gradually and noticeably. Sudden failure is unlikely. Most engines will operate on minimum oil quantities / qualities for some time.
Overheating a OM15 / OX26 mix may produce cockpit fumes which over a long term may cause health problems, in the short term, the smell would be apparent enough for oxygen to be called for.
The RAF has a "No Blame Culture" and should this be degraded many of the contamination confessions before flight would have been hushed up and no one would ever know.