Incorrect highlighting of spelling errors

Why does Trados Studio sometimes underline words as spelling errors when there are no errors?

It happens randomly. It mostly happens when you have selected the MS Word Spell Checker and less frequently with the Hunspell Spell Checker. Running a spell check (F7) shows that there are no errors but the words are still underlined with the red wavy line in the Editor. The only way to get around this is to close and reopen the file in the Editor. 

Anyone else seeing this?