Adding Index Markers Causes An Overset

I have a script in IDCS2 that places index markers based on a character style. However, in some files, after placing the index markers, my layout moves and now has an overset. I'm thinking, insertion of index markers should not make my text run-off but in some cases these happens.

Is this an InDesign CS2 bug? The index marker is a zero-width character right? Does the insertion of an index marker affects how the paragraph composer works? I'm guessing this is the cause of my problem, can anybody confirm this?

Thanks,
Jeff