I've got a PDF open in Acrobat.
I like to scroll the "standard" way, where I can press the arrow key up to go up by a line or two, and the arrow key down for the opposite.
But sometimes I accidentally click the text, and then suddenly the arrow keys control moving the cursor around inside the page instead, so I have to hold it down and move the cursor all the way to the bottom of the page before it'll start scrolling the page down, and then all the way back up if I want to go up again.
I can't find any way to move focus out of the text once I've clicked it. I actually have to close Acrobat and reopen the PDF. It's kind of amazing.