I have noticed something about Ranger Andy and his hand to hand skill he teaches. If you already have the one perk that allows you to knockdown an opponent you will receive his training and the screen goes black, upon its return it gives you his training pop up message again then locks you out of all controls save for the escape key. I have tested this twice with 2 different characters that have pointed the knockdown perk prior to reaching Ranger Andy, both occasions I had to reload old saves and not do his side quest.

If this has already been posted I apologize, I did a search but did not see anything regarding this issue.

I found this effect too, the other day, but the reason I thought it occured was if you recieve it before as the two conflict, and I'm sure I did not get any perk before hand.
Your idea makes sense as well, if not more so, be fun to find out what makes it happen though.

I received the Ranger Takedown after taking Super Slam and I didn't have any problems - or where you referring to a different knockdown perk?

I went back and tested it, the two of you are correct...making the speech check and obtaining the training prior to being handed the letter triggers the bug.