See attached screenshot.
Created attachment 534409 [details]
I'm not sure what causes it to appear, but once it appears, the only way to get it to away is a hard refresh of the page.
I think this one could be platform/browser specific. I have never seen this on a mac.
After talking with Ian, looks like this one has gone away (possibly with smartgwt 3.0).
I have seen that in the past on OS/X as well (there may even be a bug report on it )
IIrc that was some debugging information left over by Greg -- perhaps to support Selenium in detecting when the page was finally loaded or such.