There are a number of common word pairs I see every day that are swapped one for the other. Sometimes it's just confusion on the writer's part, but I can think of at least one case that has more to do with precedent. When something is permitted—and even encouraged—for so long that it becomes commonplace, it's difficult to point to it as an example of linguistic error. Farther and further are two words frequently used interchangeably, and although there are plenty of historical examples, following them only adds ambiguity to an already suffering language.
Farther should be used in connection with physical or chronological distance, while further has more to do with movement or distance in an intangible sense.
We had to drive farther than we planned.
As we move farther into the future, we begin to recognize the past.
In this way, we further the cause of justice.
Nothing could be further from the truth.
As you can see, the first two sentences are about the sort of distance you could measure with an instrument of some sort—an odometer and chronometer in this case. It's about time; it's about space. (If that triggered the memory of a very old TV show, you have my condolences.) In the third sentence, you could substitute the word promote for further; the meaning would remain the same. Further is often used in the context of some kind of abstract increase. In the fourth, the word is used to indicate a matter of degree; you could use a phrase like more removed instead.
Two more words I see swapped around all the time are affect and effect. An easy way to remember the difference between these two is to think of the phrase cause and effect. In the alphabet, a comes before e, and cause always occurs before effect.
Your actions will affect the outcome of the game.
What effect will your actions have?
In the first example, you could substitute the word influence for affect and still have the same meaning. In the second, you could use result in place of effect, thereby keeping the original intent.
Really, there are quite a few definitions for either of these words that have nothing to do with cause and effect, but this is the most common substitution blunder I've seen associated with these two words. You can take your personal effects with you when you go on vacation, and your therapist may have noted your flat affect, but the definitions related to influence and result are the two that cause problems every day.
Of course, when it comes to material like ad copy, all bets are off. While I don't think it's necessary to butcher the language, I understand that the perception is the reality; you have to make certain concessions in the name of common understanding. But aside from that, I think there's enough ambiguity and general language destruction out there as it is. We don't have to help.