Most of the time for us its a payment problem - the customer has forgotten to use the correct reference on their bank transfer when paying us, or an email issue.
This is mostly teething issues, like removing old email accounts creating pop3 login errors when they close their old isp service after connecting to us etc.
Its not that often that we have equipment or hardware failures - of 400 customers in the last year, I have had to only replace one rooftop radio, and three routers. The three routers that failed were all due to the power supply failing.