Hi, there! Indeed it's been a while.
When I started doing mods, I remember that happened, but, I don't remember now what is it. I know the "invisible" floor with debris in your picture could be a loose that was marked as "floor" on the editor. But, you said is a switch, so is a button to open or close a door. I don't know what or how your friend did that.
Unfortunately, I know little about those things. I just use the editor to create the levels, nothing special. Other than that, is to change "index" to make little and half open doors or to modifier the way chomper and crushers will act. And I just copy and paste the hex stuff other modders posted. You'll need to ask that on tutorial threads.
To change (export/import) levels, could be the problem. I did that for my mods, but always I select "Level": "Clear all rooms" and "Change type" to avoid things like this:
viewtopic.php?f=121&t=3508
On the editor won't show problems, but in the game, maybe will.
Also, your friend could had changed: "Object of current tile" and "Attributes of current tile". Maybe "Door events" (index), too.
There're rooms with special events that, if you won't use those events, you can use another room or simple, disable all events on the editor.
No. Once I said here that I wanted to do 2 more mods. But nothing was done. Then, I thought about make just one more, but, I didn't move one "tile", hahaha. I don't know if I gonna make it. Too lazy
I understood what you said. The Google translator also translated pretty well the Chinese Language.
And here is a simple pic of a loose with "floor" marked on Attributes of current tile:
Instead the loose to fall, it stay there and become debris in a "invisible" floor. About the wall, I don't remember how that happened to me.
May the Moon light your path.