One of my personal favorites is Koolance's wonderfull temperature monitoring, which has caused me to have many arguments with people in the past.
The probe, which they either say or imply, is the CPU temp, actually measures water temp ( have I got that right? ) and is therefore MILES off the actual CPU temp.
I was arguing with a guy who had a reading on his Koolance probe of 27 degrees, on an overclocked Athlon. He was mocking me because my home made effort ( DIY waterblock, Eheim 1048 and 2 foot x 3 foot car radiator ) was at 40.
I took great pains to point out the inaccuracy of the temp monitor he was using, and pointed out that how could his system outperform mine, when his pump was smaller, his radiator was smaller, he had less airflow, and the original Koolance waterblock, which was aluminium, whereas I was using a maze1 copy in solid copper.
He didn't believe a word of it, and is still running around recommending Koolance to everyone, and I bet half of them are buying it