spell checking dictionary contains words it can't see are correct

Binary package hint: firefox-3.5

The spell checker highlights "etc" as misspelled. If you right click on it, one of the suggestions is "etc." (with the full stop). This is the case even if "etc" is followed by ".". In other words, it thinks "etc." is wrong despite it being in the dictionary. If you then select "etc." as a replacement, you get "etc..", and "etc" is still marked as wrong! I imagine this situation arises because it doesn't look at punctuation other than apostrophes. "Ph.D." is another example.