Reproducing this issue:

1. Use the following code (though any code reproduces this issue):
   void setup() {
   size(200, 200);
   background(0);
   text("Test", 23, 30);
   }
   void draw() {
   fill(random(0, 254), random(0, 254), random(0, 254));
   rect(40, 34, 54, 41);
   }
   
2. Add a break point at both fill and rect lines in draw().
3. Debug.
4. When the debugger pauses at one of the break points, remove the other one.
5. Now, toggle the current breakpoint, and continue. It isn't possible to add a break point after this.