OK, another weird problem with my new game.
On some rare computers only, on some levels, the player falls through the ground when the level begins. It's as if the player object is created before the others, and so has time to fall through the terrain. In one level that meant the player was underneath the ground; in another, where you lose if you fall out of the level, he was destroyed, but then was actually in the right place when respawning (which involves leaving the layout and coming back).
Now, at first I thought it had to do with how "thin" (just 3 tiles) the ground was. When I made the ground thicker in one of these levels, it seemed to work (I also moved all the tiles upwards a bit). But in another level, adding more ground underneath the player did nothing at all.
I've tried disabling the platform behaviour when the layout starts and waiting 300 milliseconds before re-enabling it, which seems to have made it so that at least respawning works. Before it just kept spawning the player the wrong way.
So... what could be going on here?