…is graduate school, argues Derek Lowe at Chemistry World:
[J]ust as lab hours per calendar day (and perhaps also hours worked alone) are at their highest, one’s own experience is nowhere near at its peak. Graduate school is where chemists encounter a lot of reagents and procedures for the first time, and not all of these encounters will go smoothly. This is when one might find out, for example, just how remarkably air-sensitive trimethylaluminium is (if there isn’t a flame burning from the end of the syringe needle, the bottle has probably gone off), or just how long a large aqueous phosphorus oxychloride workup can sit around before it suddenly erupts all over the inside of a fume hood (several hours, damn it all). …
Almost everyone who makes it through to a doctoral degree will have had a chance to appreciate the saying that nothing can be made foolproof, because the fools are too ingenious.
In addition to Lowe’s factors–long hours in the lab, inexperience, and being surrounded by inexperienced people–I would add that graduate students are largely of an age when they feel invincible.