the issue reported by phred was the close button being painted on the left of the header bar when opening a dialog window
I had the close button painted on the left of the caption bar in the main panel.
same position as phred, in line with the RH stop button, but his was on the header of a dialog window - mine was on the main window.
On the skin I was using it was immediately recognisable as the highlight close button.
If you look closely at the image provided by phred, it appears to be the highlight close button as well.
Is this the very first time this has happened to you and was it immediately after startup?
Yes. Both for first time and immediately after startup.
Working on a new skin and viewing in 2.5
added splash screen and a few new left sidebar icon overrides in the .xml
restarted 2.5 with toolbar button to view splash screen
closed 2.5 and then started 3.0
LH close button (highlight image) appeared
re-started MB and not seen since.