power failure - UPS and separate distribution board
Chokage of pipe - ensure no object can enter the pipe opening. Filtering for your overflow and double returns, etc.
Loose Connection - Fastener and regular check.
Faulty Equipment - Standby equipment
Flood - can cause by chokage, faulty equipment, loose connection, etc. Plan your sump and ensure when everthing stopped, your sump can manage. For your return pipe, drip hole to prevent flooding due to back siphon of water from your return pump.
Death - sea cucumber, coral such as anemon, etc can be sucked by wave maker, causing water to be contaminated
Overdosage of supplement - equipment such as CR, dosing pump, etc.
Temperature increase due to breakdown of chiller and lighting(especially MH) turned on.
Murphy law - anything can happen, leakage of tank, etc.
Just do regular check and be prepare for worst case. In reefing, there are systems available to prevent this and that but there is no 100% assurance. Regular check and maintenance helps.