48. Box checkpoints.
I actually disagree with this one. I know where you're coming from: starting at one of these in the editor crashes the game, as does loading a saved game at one. But in practice, the chances of someone actually saving at one of these are quite low, especially if used in an area full of gravity lines and/or crumbling platforms.
Related:
51. Checkpoints on crumbling platforms. You may have noticed that after respawning at one of these a few times, the platforms look like conveyors. What's less well known is that if this process continues, it ultimately crashes the game.
52. Checkpoints on the crumbling single blocks available via Direct Mode. I've never seen this done, and for good reason. Unlike the regular crumbling platforms, these don't come back when

respawns.