Surprises in diligence
An undisclosed dispute, a customer who left last month, a lease expiring in ninety days, tax arrears. Any of these can be handled — unless the buyer finds them rather than being told.
Discovery erodes trust far more than the underlying problem does.
Drift
Deals lose momentum. Six months in, the seller is tired, trading has dipped, advisers are re-opening settled points and both sides start wondering whether it is worth it.
Set a timetable at heads of terms and treat slipping dates as a warning sign, not an inconvenience.
Second thoughts nobody voiced
Sometimes the seller was not ready. That is legitimate — but it is much kinder to say so in week two than in week twenty.
The other classic is trading falling away during the process because the owner stopped selling. Keep running the business as if the deal will not happen.