This assert has been triggered in 2 different ways but is probably the same underlying problem. Environment was dual processor, winxp, with a number of graphite fonts installed. Running openoffice.org.sil 2.4rc6-1.
Case 1. Scrolling (or paging) rapidly through the dont selection drop down early in the session.
Case 2. Scrolling (or paging) rapidly through the "Insert>Special Characters" menu while displayong a graphite font.
Stack traces of both caes available from the submitter.