I run into these occasionally when flying or leaving a dream scape. My wife reported the same from a LD she had last night,though we've never discussed this type of content before.

I am wondering if others here run into this unexpected obstacle ? What do you think it is or represents ?

My opinion:
In one of my LDs I once grabbed one believing nothing would happen(being 'my' dream and all ) - I was actually shocked!...LOL. It wasn't that bad though,I kept going about my business. Other times while flying (even with a car )nothing has happened except for loss of apparent altitude. I think they may represent,or act as a boundary.