I learned that in RL, going ahead flank with my periscope up while submerged would prolly break it right off. And then there's the vibrations that make it essentially useless at anything between that and slow ahead. Now when I play I can't go faster than 2-3 knots with 'er up or I feel like I'm cheating.
With that in mind I feel compelled to say, once again, THANKS FOR NOTHING SAILOR STEVE.
I also learned about the differences between the observation 'scope and the attack 'scope, and why/when one would be preferable to the other in various situations.
And about a million other things.