We left one behind once. The Mels covered it if I remember correctly, but only just. The aircraft had just come out of maintenance and from memory (it was a long time ago!) the EECs were MELed out and after starting both engines there were intermittent indications on one of the of the EPR gauges as well as one of the fuel flow gauges-or something like that. Can't remember the exact details but basically it was something to do with the engines that the mels covered, however, the engineers had no idea as to the cause of the combination and it was all too dodgy so we left it behind for it to get properly fixed and went to another plane! As far as I know no ramifications, just the sensible safe thing to do. Doesn't happen very often though! (Not dispatching cause of mels/too many mels I mean...)