[news.eclipse.newcomer] Doesn't stop on break points (using Debug, not R

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> From time to time breakpoints does not work in my Eclipse/Tomcat projects, 
that is, Eclipse does not stop on them when run in debug mode.

There is a visual clue of what the problem might be: usually active 
breakpoints are indicated with a blue dot with a checkmark on (√). 
However, when I have this problem there is no checkmark on the (still 
solid blue) dots indicating a breakpoint.

I have tried cleaning/rebuilding the project, restarting Eclipse, adding 
the breakpoints when Tomcat is up/down, all to no avail.

Windows XP 32, Eclipse 3.4.

Anyone that know what the problem might be? Googling on "eclipse 
breakpoint problem" mostly returns discussions about Debug/Run or missing 
line numbers.