Not stepping back and thinking strategically about where to go and why before beginning a PhD program.

Because of a deteriorating relationship and problems in my current lab, I felt completely overwhelmed by the prospect of choosing a PhD. I was being told by my superiors that I had loads of potential and therefore had to get into the most prestigious institute possible. I therefore copped out of the difficult task of doing a PhD and essentially allowed my supervisor (who I knew was incompetent) to choose one for me. I never asked myself if I really wanted to do the research I was being pushed towards, if labwork was a rational choice for someone with my skillsets, or, more importantly, if I wanted to do a PhD at all.

Imagine my embarrassment when I start reading lesswrong and discover that stories like this about grad school are a stock example of irrational behavior...