A few weeks ago I missed a key deadline. I missed the deadline because I thought my project was 4 weeks long and in fact it was only three. The deadline was essentially driven by a fixed cost engagement. It wasn't so much a schedule deadline as much as a number of day effort deadline.
My response, when I earned of it two days before the actual deadline was to rush, to work long days and try to recover.
The result, as expected: quality deteriorated, I made mistakes which had to be fixed and I ended up delivering 2 days later than I was originally planning. I would have been better off just maintaining my pace and I would have had a better product earlier.
I know better than to do this but I still make mistakes.
Why did I go wrong? I mis-read the brief and didn't check back with anyone about my work plan. The solution is the usual one: Communicate, communicate and communicate. Then communicate some more.