"Leak" may be the right word for some configurations with conservative GC. A "leak-free" GC would presumably guarantee eventual collection of all dead objects, which is impossible with a conservative GC in theory, and in practice it's not especially difficult to get into a configuration with the Boehm GC where the lifetime of a dead object is infinite (usually because it doesn't move and some binary data that the GC thinks might be a pointer doesn't go away, because it's texture data or something).