Measuring nothing (with great accuracy)
The weight of a television set has nothing at all to do with the clarity of its picture. Even if you measure to a tenth of a gram, this precise data is useless.
Some people measure stereo equipment using fancy charts and graphs, even though the charts and graphs say little or nothing about how it actually sounds.
A person's Klout score or the number of Twitter followers she has probably doesn't have a lot to do with how much influence she actually has, even if you measure it quite carefully.
You can't tell if a book is any good by the number of words it contains, even though it's quite easy and direct to measure this.
We keep coming up with new things to measure (like processor speed, heat output, column inches) but it's pretty rare that those measurements are actually a proxy for the impact or quality we care about. It takes a lot of guts to stop measuring things that are measurable, and even more guts to create things that don't measure well by conventional means.