You can throw the key though the pipe-wall into lava, and lose it forever. (Aka it does not re-spawn.)
First way i did it: throw it down to the yoshi's, and use one to spit through the wall.
Second i just throw the spring though the wall...
It's far to easy to throw something through a wall, and burn it. http://prntscr.com/5ms29h (key)
or just lose it. http://prntscr.com/5ms44o (spring no yoshi. Held down as well i think.)
Bug2: Pic1: http://prntscr.com/5mrsm2 Pic2: http://prntscr.com/5mrt25 Killed the ghost with a key. (Just threw it at it)
I then went over to see if i could re-spawn the ghost (walking off screen to the left, and back on)
but before that i went over the wall, jumped down, and (held run + spin) landed on the red yoshi, and let go of the key.
Then me, and the red yoshi somehow teleported over to the other side. (key re-spawned when it got off screen)
Bug3: This is a worse one. I enter "A World One Level" and crash with this popping up: http://prntscr.com/5msqrj
Running as admin makes my old saves disappear btw. (new ones appear if i save though,
and stay as long as i use the program in admin mode. Same i'll get the old ones back if i start it in none-admin mode)
I don't even know if you care about bug like these. But i wanted to at least note that they are there. (Well i hope you note the crashing one)
Also found a little hidden box: http://prntscr.com/5msnuw