TkTextGetIndexFromObj has a severe bug which is still undetected. See this extract:
It is possible that objv[1] and objv[2] are referring the same object, and in this case it is possible that inside the call of TkTextGetIndexFromObj(interp, textPtr, objv[2]) the memory of the pointer index1Ptr will be destroyed, and the application might crash.
This bug cannot happen in the revised implementation, because the caching of indices has been removed (no benefit since the introduction of the section structure).