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Details:

An error in a filter would cause an infinite loop.
ibuffer-update calls ibuffer-redisplay-engine which calls
ibuffer-update which calls ibuffer-redisplay-engine, etc.

ibuffer-generate-filter-groups calls ibuffer-included-in-filters-p.
If ibuffer-included-in-filter-p-1 meets an unknown element in the
filter, it calls ibuffer-filter-disable, which calls ibuffer-update.
If the filters have not been properly disabled, we go round and round.