I once had to develop a full application with really hard problem solving in a really complex domain that I had hard time to understand. This was for a big national phone operator.
We delivered it under a lot of pressure from the sales team (I was in a shitty company, with few employees) and the contract was in multiple millions euros. That was literally one of our 2 or 3 customer.
We delivered. One day we eventually get paid (our customer was well known in the industry to pay only when they are forced to - yeah, shitty customer too).
The project is finished, customer is happy with what we demoed and delivered.
Then maybe a year later, we decided to add a feature to this product to be able to sell it to another client. I got pull and build this damn thing. Nothing worked. Like, nothing. The login form was broken.
Then I remember. A full year and not a single support ticket. The company owner « joking » about how our sales team used alcohol, parties and embarrassing photos to make their customer buy our software. The insane amount of pressure we had to deliver this technically hard project.
I left this company the day this popped in my head.