I sometimes came across the issue that i have code like this:

void blabla() {
  System.out.println(); <<-breakpoint here
}

Now one modifies the code (e.g. delete the line and save) it sometimes happen to be in that state:

void blabla() {
} <<-breakpoint here

I' not 100% sure how it happens, but in that case JDT halts at that line (maybe when return from the method?) but one can no longer remove the breakpoint (as one cal also not add one) through double click on the ruler. It only works to find the breakpoint in the breakpoints view and delete it there.