Personnel breakdown: illness; overload; stress; incompetence; incompatibility; language and cultural issues.
Organisational breakdown: poor planning; insufficient staff; lack of support from conference organisers; unclarity on duties and roles; "holes" in responsibilities.
It is wise to expect some breakdowns; this means you will insure yourself with safeguards and backup functions to the extent you can. Ask "What if ... " questions to focus on alternatives that you could fall back on if the unexpected or unthinkable happened.
Another good general practice is to keep your system design as simple - and appropriate to the environment - as is possible.