I'm not an expert on this - I've only used transcripts. I hope the experts will be along later, but in the meantime here are some ideas.
The entry depends on several things - not everybody could write their name, but would make their mark; sometimes the witnesses were the churchwardens and I would guess a vicar wouldn't always trouble the churchwardens with actually signing the register. Some vicars were more conscientious than others! There is a huge difference in what was recorded, sometimes you get the occupations, sometimes you don't, sometimes the fathers' names, sometimes whether the marriage took place by licence or banns. If by banns then there 'should' be a record of that, but as you hint, the bride and groom may have had last minute doubts. There is also a set of records called the Bishop's Transcripts, which was a copy of the register sent off to the Bishop. I wouldn't think there would be a corresponding entry if the marriage failed to take place. If they still exist. In some areas only the Bishop's Transcripts have survived, better than nothing, but as in all copies, prone to error.
If I'd been the vicar and the marriage didn't actually take place I would have made a note to say so, or crossed it all out. So, hard to answer your question!