There once approached me a client, with a request to be done. Here is a recap, with emphasis on time limits.

C: Ok, so we need this and this thing to be done that and that way...
*short talk about technical side of the project, unimportant to the rant*
C: Can it be done by 25th, this month? (It was 4th of the month)
M: No way, it'll take at least a week more, so realistically I'd say around 7th next month.
C (Had no option but to agree on the date)
*we arrange the price as well (was not a bad one at all)*

So I started working on the thing and one night, about a week or so in, I probably had a cup of tea too much, I suddenly have a breakthrough. I sat behind a computer from 22:00 till 17:00 next day, nonstop. I didn't even eat anything in the meantime. The project was far from done, but I did quite a lot of work. Anyhow, when I have completed the project, not only was I not over the deadline, it was 22nd of the month, so even before the wanted time! When I contacted the client and told him that I am done, he was ... let's just say very happy. The deployment went fine, but when I checked my bank account, for the payment, there was a surprise waiting for me. The number was 25% more than what we have arranged! Me, believing that it was a mistake, immediately messaged him about it and he responded:

No, this is just a small gift for you, because you finished that quickly.

(and not to forget, I have coded things for way less than those 25% and was completely fine with the price, so it was not a small amount)