I distinctly recall a situation where speed made a huge difference. I took an undergraduate lab at UCSC where you were given raw bacterial cells and had to use the original literature to purify restriction endonucleases with it. The course ran for a quarter, and on the first day the professor said: "Most of you are going to spend a few days planning, then running your purifications for a few weeks. If I were going to do this, I'd spend a couple weeks planning and then do the whole experiment in a day, because the moment you break the cells open, all the proteases will start eating up your restriction endonucleases and you will lose activity in a couple days.
I've also found a few situations where I couldn't get somethign to work and it turned out the person who had it "working" wasn't running controls (positive or negative), so you didn't really know if it worked.