1) In any app (using GtkFileChooserDialog to save: an example is gedit), choose "Save".
2) Click the "Browse for other folders" expander
3) Click "Create Folder"
4) type in a name such as "a/b"
5) The following error message is shown:

"The folder could not be created", "Error creating directory: no such file or directory"


For reference, if instead such a name is chosen for a new folder in Nautilus, the following, right, message, is shown:

"The item could not be renamed", "Sorry, could not rename "untitled folder" to "a/b": Slashes are not allowed in filenames".

(evidently, in the FileChooserDialog, the presence of "/" must be catched)



Another way to see it is that if instead a folder "a" already exists, executing operations 1)-4) creates a folder "b" _inside_ "a", which again is not what an user would expect.