In a scientific experiment,
(1) if something can go wrong, it will do so just before your grant is up for review;
(2) if the reading on your detector is correct, then you have forgot to plug it in;
(3) if several things can go wrong then they will do so all at the same time;
(4) if nothing can go wrong with your experiment, something still will;
(5) left unto itself, your experiment will go from bad to worse; on the other hand, if you pay attention to the experiment then it will take three times longer to complete than you thought it would;
(6) Nature is both subtle and malicious (Murphy stole this one from Albert Einstein);
(7) a straight line will never fit your data, and using a wiggly line will result in the rejection by referees of the publication of work;
(8) if you make a great discovery today, you will find a major error in your methods tomorrow (experienced experimentalists call this effect "here today, gone tomorrow");
(9) in contrast to a radio, banging your apparatus when you are at peak frustration will not fix it but permanently break it (for this reason, it is important for experimentalists to remain calm at all times);
(10) when your experiment is just about to succeed, you will run out of grant money.

In short, in a scientific experiment, anything that can go wrong will go wrong.

By the way, the staff of Jupiter Scientific has a proof of Murphy's Law but it is too long to fit in the margin of a webpage.

Source:  http://www.jupiterscientific.org/sciinfo/jokes/miscellaneousjokes.html