By giving you an endless spinning wheel instead of just telling you what the issue is?

I've had this while trying to install apps on an iPad (you need a payment method if the free app you're downloading has extra paid stuff you could buy from it, but why would they need to tell you that) and with their TV+ service where downloading too many (how much? who knows) things at the same time, like you're about to board a long haul flight, gets them stuck in a loop without downloading.

Another one of my favourites is "A USB device is consuming too much power, try disconnecting and reconnecting it" without any way of identifying which device it is.