You will over-insert a Glock 17 spare mag in your 19 and this will get you killed in a fight for your life! I've heard more than a few folks spout this one, in person and on the Interwebz. Here's my response. Enjoy!
http://www.youtube.com/watch?v=DXohG9HfR1M
http://www.youtube.com/watch?v=DXohG9HfR1M