I made my object with a cylinder and one block that was doubled during mirror modeling.
Now i am in the situation of the image. If i try to delete any of the selected faces on the left, it deletes the whole mesh. What the heck?
I managed to actually delete one of them by selecting the uv and pressing del, but if i do it with any other face, it behaves as described. I tried to close and reopen BlockBench, but in vain