Re: How many?
I don’t recall many customers overly concerned, but I always advise them not to pull the dishwasher out if they ask.
A quick Google search reveals that tar can start to melt at around 50 deg C. Since most dishwashers reach between 60 and 70 degrees in the final rinse, its no surprise the tar based backing will soften over time and stick to the plug.
We all know though that it’s a stupid place to install a plug, particularly for integrated appliances, as the plug (and for that matter taps and waste) should be located to allow easy access for safe isolation and maintenance.