I've watched people swim through Mario 64 by mashing B. Everyone should do that now.
I never got what that action was for. I guess you could make a point about swimming by holding down A as well.
Remember the dark, dark days where you couldn't run in pokemon games? And then when you could run but not indoors?
Ugh... I fell in love with HG/SS's run toggle, but for some inexplicable reason they later games lacked that option. I think toggleable walk+B->run and run+B->walk options would be better. I honestly don't get why most console games don't allow you to customize button actions. Super Metroid did it, and Sakurai finally let us map the input for Brawl.
Talking about Brawl, that reminds me about the incredibly useful tap jump toggle. Avoiding randomly jumping and making up tilts easier to do (as well as pulling off up aerials without wasting your second jump), but due to it I now play badly on earlier games unless I'm double jump cancelling Ness' aerials.
Or when people don't reverse through racing games. It's a minor thing but it makes me so annoyed when I see somebody do that.
My cousin and I coined a special term for driving in reverse after crashing to quickly get back on track, or alternatively driving on reverse until the next turn (we did this in a top-down game where the car's reverse speeds were nearly as good as their regular speeds). We named the technique as a homage to that scene from the first Austin Powers movie where Austin repeatedly switches gears from forward to reverse ones in order to turn what looked like a golf cart around in an extremely narrow passageway.
That reminds me of how I used to jump in Mario Kart 64's DK Raceway just as a coconut was about to hit me to be sent flying slightly farther (I think), but there I often used A+B to turn around whenever I hit a dead end, like the guardrails to the sides of the bridge entrance in that very track.
To get back on topic, I don't remember since when I started running constantly (as well as long jumping in 3D Mario games that support it), but I do remember being unable to beat Super Mario Bros. because of a particularily long jump in world 8 where you had to land on a lone block and jump again through a smaller pit.