When there is any inline box in the line, and the line ends with an unbreakable text frame and a non-inline element, the last non-inline element will trigger multiple backing up warning because the line layout always records a line break opportunity after it.

Open this testcase in debug build, and change the width of the window, there will be a lot of lines like: [7728] WARNING: We shouldn't be backing up more than once! Someone must have set a break opportunity beyond the available width, even though there were better break opportunities before it: file c:\mozilla-source\central\layout\generic\nsBlockFrame.cpp, line 3823