I created bugs 374925 and 374929. In the process I created lots of duplicate bugs, because I added an attachment, but no method to detect the content type was selected. Bugzilla generated a big red error, but created the bug anyway without the attachment. This I didn't noticed and created bugs until I successfully added the attachment.
Now a description:
I did never notice that a lot of bugs where created while the errors where displayed and during the process I was quite sure that the bug creation process was canceled due to the error, due to the hint with trying it again.
FWICS currently Bugzilla just displays the newly-created bug with attachment creation failure message above, so it should be pretty self-evident that the bug is created.