We've had several delays in the past, and they haven't been serious. One year we were delayed because our plane was coming from Jamaica, and a hurricane was sitting right on Jamaica, so planes were grounded for several hours, meaning our plane was delayed in leaving there, and thus arriving in the UK.
Then, another year, they had difficulties with the entertainment system and were trying to fix it, but it wasn't working. We were all boarded, although late, and they were still trying to fix it while all the loading sheets and stuff were done.
In the end, the pilot made the decision to stop with the entertainment system fixing, and made an announcement to the cabin saying he thought we would all prefer to get going to Orlando and suffer the few channels they had working than wait around until all the channels were fix, which could be hours. Everyone cheered, so he made the right decision there!
