When there's a change made to a question or answer that you're viewing, a handy bar pops up saying so, with a link to refresh the page to show the updated content. On meta, this bar has the text color and background color the same, so it appears to just be a strange grey bar. Here's a picture:
1 Answer
This and the "N questions with new activity" bars will be fixed in the next build, > rev 2012.5.4.2454.
/questions
page.