One of the logical corollaries to Murphy's Law is known as the Harvard Rule of Animal Behaviour. It goes something along the lines of, "You can have the most beautifully designed experiment with the most carefully controlled variables, and the animal will do what it damn well pleases."
That is currently one of my biggest problems. I changed my entire teaching schedule on the idea that the ascidian species I was working with last year would be back again this year.
These little babies just have not shown up again. Whether it's En Nino, sunspots, bad luck, or whatever, I'm sort of stuck without the animals I wanted to work with. 'Tis a quandry. It wouldn't be so bad if I didn't have a student who worked with me last year ready to do a series of follow-up experiments. Eeep! So now I have to think of a back-up plan.
Still, I find this problem less aggravating than my other major problem, which is the seeming inability of getting anything I order here promptly.