Sometimes, things don't work out as planned. For example, last week my Microbiology lab set up an experiment to test whether sulfa drugs work by competitive or non-competitive inhibition of a crucial enzyme. The hypothesis was that sulfa drugs work by competitive inhibition, and previous experiments have been run to show this mechanism of inhibition. Unfortunately, no one obtained the expected results. Now what?

I always thought that doing the successful experiment was the hard part of research. However, now I realize that the most difficult thing sometimes is to figure out exactly what went wrong. Why did the scheme that you set up not work according to plan? This takes the most effort and introspection.