Most difficult levels in existence?

[AdrenalinDragon]

Call me crazy, but I actually like Exit Chip. Although this might be more due to memories of first discovering/solving the level, plus the fact that I don't think I've even played it since 2002, back when invalid tiles were still cool...

To elaborate a bit on the memories - when I first discovered the level in EricS1 and read the hint (not present in the CCLP2 version) that read something like "There's no way you can possibly beat this level, but I can - muahahaha!" and after playing it for a while and finding that the obvious chips were booby-trapped, I actually thought the hint was serious (or at least the first half of it) and that Eric meant that the level was unsolvable... I'm not even sure if I figured it out it was solvable until CCLP2's release, at which point I realized it must be... Even with all the hidden items, it was still a good feeling to finally figure out the solution that I once thought didn't exist...

I must admit, playing through this levels back in 2002 must have been quite an experience. I could see levels like Exit Chip and Warehouse II being difficult without solutions, and I've always wondered, how long have invalid tiles existed in Chip's Challenge? Were editors made to allow them or was the game allowed to do this from the start?