The engine crashes if you don't do a "save as".
ie. If you run the level as untitled.wmb it crashes the engine.
It's OK after you "save as" something else and build and run.

In A7.07 you could run as untitled.wmb, you just got a warning, but it still previewed the level/map_ent.

As far as the texture lock problem, we tried this on 2 machines the other day, and It failed on both.
I was sure we were using 7.50.
I am unable to get it to fail again, sorry for false alarm if there is nothing wrong.

I'll let you know if we can re-produce it again, consistently.

Jethro.