After some time I was quite familiar with their stack and had gathered considerable domain experience. This led to an idea how to halve the database load (and the cost would presumably fall by a similar percentage), which I wanted to use as leverage during contract renegotiation.
I boldly offered to work for free to halve their database load, in exchange for being paid half the money this optimization would save over the course of one year. This would basically triple my pay, and they would still save money.
They declined, and I moved to a better opportunity.
Last I heard they had to pay a team of 4 new consultants for a year to implement the same idea I had. Without the domain knowledge, consultants couldn't progress as fast as I suspect I could have done (my estimated was 2 months of work).
I know it's very petty, but I regret revealing too many implementation details of the idea during the pitch and allowing the company to contract other consultants to see it done.