Greetings Geezer (and, incidentally, the rest of the forum). Part of me can't find myself staying terribly long here but I had to sign up and comment on this game. First off, wow, it is probably the best fangame I've played (though I've only played, err, Fangame, and seen portions of GB). Kudos to you sir for your Excellent® work.
I am here primarily to report a glitch though, and I'm not sure what happened. After Chapter 1's boss (I'm not spoilertagging one name), I seemed to encounter a glitch that prevented me from moving onto Chapter 2. It might've been my own fault, but I am unsure if my actions caused it or not. I was booted to the map screen, and noticed that I was missing 3 Coins from Chapter 1, so I went back in and began from the boss I just beat. On exiting to the map again, discovering I was unable to backtrack to the rest of the chapter to find the Coins, I found myself unable to move onto Chapter 2. I'm thinking that maybe me going back into chapter 1 before stepping foot into Chapter 2 caused this, but I can't be sure. I had to beat the boss again to move onto Chapter 2, though it wasn't a big task seeing as how I had mastered it by that point.
As an aside, I discovered that I was able to just barely get into the next screen in the following image the wrong way (spoiler tagged due to giving tip for a Coin). I triggered the spike to fall and kept hugging the wall, barely tapped jump and made it between the spike and the much tinier spike, allowing me access into the next screen. on completion of course I died instantly since presumably I'm not supposed to be there (and since that portion itself leads to one Coin and there's no way you could get there in the same life, I can only assume it's an out of bounds exploit given I seem to die by being in a wall block). I even did it 3 more times to get a good screenshot of the result. Guess all that nets me is bragging rights

One last thing - again, kudos, and one small thing I enjoy that you did compared to the original and Fangame is making the music not restart on death, it leaves off where it did when the Game Over music cuts in.
Edit: Another out of bounds exploit in the spoiler tags, second two pics.