I just encountered a bug in Super Mario Bros. The original super mario bros for the NES (well, ported to the GBA, but still).
Basically, I'd been playing through and when I reached world 2 I had mario in his super form. So off I ambled, using my mighty pumber boots to crush goombas until I got to the first powerup block.
Which produced a super mushroom rather than the expected fire flower.

Huh?

And even more baffling, when I picked up the 'shroom I got the beoop-beoop-beoop sound, and mario flickered... but when he was done he was normal, tiny mario again.

What?

Tumbling further down the rabbit hole, but trying to justify what had just happened as a random glitch I carried on my merry way. A few levels down the line however, I took damage for the first time as lil mario collided with a spinning fire wall.

Oh no! Wait... what?

You see, rather than dying as he should, the little mario went through through the 'oh no, you've been stupid and now I'm shrinking back to normal' animation. Which was odd in part because he was already in his normal form, and also because at the end of it he was super mario again.

So... mushrooms shrink him, but damage sends him back to super form.
Wut.

Maybe I'd found a secret masocist mode. Maybe the coding had a brain fart. In any case this continued for several levels until I died, and then the normal chain of normal -> super -> fire powerups returned.

Freaky.