Note that debug-on-error is disabled for process filter functions and post-command hooks:

Quitting is suppressed while running ‘pre-command-hook’ and

‘post-command-hook’. If an error happens while executing one of these

hooks, it does not terminate execution of the hook; instead the error is

silenced and the function in which the error occurred is removed from

the hook

You can turn error debugging back for culprit functions; see this gist.