If I rapidly (via lua) remove and replace a combat tracker entry - at the same init (position in CT) - when there are enough entries to have an active scroll bar present, I can pretty much guarantee that I'll end up with an empty place where that entry is. Scrolling the CT immediately makes it appear.

How to duplicate:
In lua use addNPC() to add an entry into CT at same init as another entry which you then delete. If the scroll bar is active in CT - it will leave a perfect gap in the list to fit your new entry.

How to work around:
Simply scroll the CT tracker and it will instantly pop into that place.

It will never happen when you can fit all your CT entries on the page (no scroll bar).