One went off over 3 mins early at Salisbury last year.
There was a rule change a few years ago, before which if a race went off early it was declared void, presumably because of technological limitations some bookmakers may not have been aware of the early start and the position could have been exploited.
Nowadays where probably 99.9% of bookmakers have live feeds it is not so big an issue and, more pragmatically, the result is allowed to stand.
There is always an enquiry when there are early starts and the starter can be disciplined.
In the case of the Salisbury race it was a large field juvenile sprint with many debutants. They began loading fairy early to allow for any problems with runners going in – yet, unusually, every single runner walked into the stalls without the slightest resistance. The starter then had the dilema of does he let them go early or does he hold them in the stalls for another three minutes.
Of course sods law dictates if he delayed loading longer, then one or two would have played up and he would have been moaned at for a late start.