I am not operating on the user interface from other threads. I assume that scheduled PostCallback's are automatically removed when the TopWindow is closed, so one of my threads should be causing the problem.
Only Callbacks with id's (and id must >= and < (int)sizeof(Ctrl)) are alternatively removed. Aternatively, TimeCallback helper class callbacks are removed by TimeCallback destructor.
PTE callbacks are not removed, but they are 'emptied' when pointee destructs.