None of it was my gear.
As a contractor hired to perform services, it is expected that you will use your expertise to provide those services in a competent and safe manner. When unsafe conditions are present, you cannot fulfill that expectation. (Doing anything in an unsafe manner is a hallmark of incompetency.)
Your due diligence in this situation is to inform the responsible party (operator of the venue, the owner of the equipment, the promoter, etc.):
- Your observations (overvoltage, undervoltage, incorrect or unsafe wiring, etc.)
- The potential impact of the condition (equipment damage, personal injury, etc.)
- Suggested corrective measures ("contact a licensed electrician")
- Actions you will and will not take ("I'm not touching this" or "I'm not connecting my equipment until corrected")
- Limitation of your liability ("I'm not qualified to rectify the problem" or "don't blame me when the lead guitarist gets electrocuted")
As this is a potentially dangerous situation, this should all be documented in writing with date, time, and location of observations. The responsible party should sign acknowledgement of receipt. (This doesn't imply responsibility, but only as proof they were informed.)
Should the worst-case scenario happen, being able to present documentation created at the time of observation carries a lot of weight in investigations and litigation.