There is a little ridge between the toolbars - when the mouse is over it it indicates that there is something You can grab.
Did You possibly left-click (grab) it & use it to drag a bar to another position?
Else the bars change position when not maximized - or just a glitch.
Things that go away before You can catch them are no real bugs
If anything can go wrong You will eventually get it reported here.
p.s. If You think You stumbled on a visual bug you can use the print button to make a screenshot & insert it in e.g. irfan viewer or another graphics program.