The font for this test case has an AAT Extended Glyph Metamorphosis Table with a single Ligature Subtable whose finite-state machine has one single state. For glyphs a and b, the transition marks the current glyph. For glyph c, the transition also marks the current glyph, and then it executes an action to substitue the marked glyphs by a ligature. When rendering the strings ac and bc, the implementation finds two glyphs on its glyph stack, which should get replaced by the corresponding ligature. When rendering the string cc, the implementation is asked to pop two glyphs from a stack that contains just one. When rendering abcc, the stack will contain three glyphs when handling the first c.