Irksome mistakes

When browsing the US tech forums, I keep coming across the same irksome mistakes time and time again, and from otherwise eloquent and skilled English-language users. My pet peeves: Confusing the words "reign" (as in "the reign of Henry VIII") and "rein" (as in "give him a free rein", "he held the horse by the reins"); "principal" (adj., as in "his principal means of sustenance") vs. "principle" (n. as in "It's a matter of principle"); and most unforgivable of all: "nucular" instead of "nuclear". It's derived from "nucleus", fer chrissakes. This ain't rocket science, people!