Thanks for sharing your experience Frank, this particular hazard is mostly unique to bottom vent stoves like your Channing, and can occur even if you don't "level the ash pan". It also can occur in any horizontal surface located very near the exit of the stove flue even in a top vent, like a horizontal run of flue pipe or a heat exchanger like the Magic Heat.
Also important to note, it is not just the exit of the stove that gets clogged, the first el after the stove flue outlet on bottom vent stoves tends to fill up in less than a season, I usually vacuum that area at least three times during a season on any bottom vent stove.
This is a good cautionary note for all users of bottom vent stoves as we enter the latter part of the season, most have been burning since late October and as we enter the fourth month a little inspection and maintenance is a good idea.
Thanks for the reminder!