Possibly one during my MSc. I had to extract protein from bacteria that were designed to produce this protein (you can trick the bugs into doing that pretty easy by adding a bit of DNA to them, coding for the protein you want). I tried a few times to get the protein out of the bacteria and had no luck, the person supervising me thought I was doing it wrong and got me to try again and again. Eventually, she did it herself and it turned out that the bug had lost the piece of DNA that would have told it to produce the protein and so I would never have been able to extract. Because it was so frustrating and took a few weeks to figure out what went wrong, it was a pretty difficult and confusing experiment.
That’s a really cool user name 🙂
I believe synthesis of a core-shell nanomaterial where I tried to cover a gold nanoparticle with copper was the most difficult one. I repeated that experiment so many times and missed one of the musical concert as well because of that experiment 🙁