When you compile a function/procedure/package and have errors and click on the errors to go to the place in the code they're from, it's opening a new tab. The new tab is linked to the old tab, because if you close it, then all tabs are closed (found that out the hard way when I lost some code); however, the new tab doesn't know about the connection the old tab was using.
I've learned to work around this by avoiding clicking on the errors when the location of the problem is obvious or just closing the file and re-opening after several tabs get open. I couldn't find where this was reported recently though so I wanted to mention it.
I've noticed this both on my Mac and on Windows 7.