The worst are the modern async UIs that become responsive early and then reset their state when they fully load - so if you e.g. already typed part of your search string, you can actually briefly see it in the textbox, and then it gets cleared away. Very common on websites, somewhat common on the desktop, in my experience.

Ironically, the old apps that just "hang" for a while during initialization handle this better, because they don't process input at all, so it all just gets queued.