HUH?!

Was just trying out my Map01DM (map01 with all the outdoor areas removed, some things changed physically, different floor/ceiling heights in places-it's excellent for DM) wad, and suddenly up comes ZDoom with a "This level contains errors that must be fixed." error. It's never, ever, in my memory, done this before, on any level, no matter what level it was that I was editing, and no matter what the errors in the level. Not only that, but after I got the first popup error, I ran error check and it didn't find any. WTF?

EDIT: And I'm using DeepSea.

Only in my first level attempt ever have i encountered the "This level contains errors that must be fixed." error. Look around it may be obvious, maybe you have a sector in a sector bug? Just a guess.

This has happened to me many times before- each time it was the same reason. As Epyo said, it's all about the sector OR a sector may be incomplete. You should make sure everything is made properly and then try building the nodes again...

I ran error check and it didn't find any

Are you sure that you ran all checks?

Try checking for unused sectors and sidedefs and removing them if there are any.

Check that the WAD is OK.

Try an external nodesbuilder.

Try opening the level in a different editor, saving, then try again in ZDoom.

Try convert it to the ZDoom/Hexen map format.

Nice topic. (i.e. make a better one next time)