Most of the technical problems that show up on the day could have been prevented during the site survey.
1. Not asking about the sound engineer
Even the best equipment is useless without someone to run it. Make sure an operator is included in the rental package.
2. Forgetting load-in time
Setup takes two to three hours before the event. A venue that only opens access an hour beforehand forces everyone to rush.
3. Ignoring the power supply
Ask what the venue's electrical capacity is. A 5000-watt system will not run off a household socket.