farther and further
Merriam-Webster's Dictionary of English Usage says that in 1906, a usage commentator named Vizetelly said, "Farther should be used to designate longitudinal distance; further to signify quantity or degree." Since then, most usage writers have simply echoed this idea, even though it wasn't based on educated usage in 1906 or at any time in the past.
MWDEU goes on to say, "Henry Bradley, editor of the F volume of the OED (published in 1897), summed up what he considered standard English practice at the time. He said that farther is usually preferred as the comparative of far, that further was used where the notion of far was absent, and that there was a large intermediate class of uses in which the choice between the two was arbitrary." This still seems to hold true today.
This is one of those pairs of words that a lot of people worry about, but for (I think) no real reason. You don't need to learn some tedious and hair-splitting rule in order to use them correctly, because you almost assuredly already are.