When you're trying to create a folder named "." or "..". Dolphin displays a warning saying "The name "." starts with a dot, so the directory will be hidden by default.", but after you click "Create directory" button and it fails to do so (obviously, you can't create these two under a *nix system), it doesn't even display an error message.
Can confirm with Dolphin 18.04.2, thanks for reporting!
I'm going to start working on this one. It should be pretty easy just to check the name of the new folder. It also seems like this problem comes from KIO.
https://phabricator.kde.org/D13805
Git commit 54298c16f37a81afb670f7b1c8fae323aedfb8a3 by Elvis Angelaccio, on behalf of Thomas Marshall. Committed on 29/07/2018 at 19:15. Pushed by elvisangelaccio into branch 'master'. Present error dialog when user tries to create directory named "." or ".." Summary: In Dolphin, when a user tries to create a folder named "." or "..", they are presented with a dialog that informs them that since the folder name they entered begins with a ".", the folder they are about to create will be hidden. This is, of course, misleading. This patch instead presents them with a similar dialog, informing them of the error and asking them for a new name. Before: {F5976307} After: {F5976315} Reviewers: #dolphin, #frameworks, ngraham, dfaure, elvisangelaccio Reviewed By: #dolphin, ngraham, elvisangelaccio Subscribers: cfeck, elvisangelaccio, dfaure, tmarshall, bruns, ngraham, kde-frameworks-devel Tags: #frameworks, #dolphin Differential Revision: https://phabricator.kde.org/D13805 M +27 -0 src/filewidgets/knewfilemenu.cpp https://commits.kde.org/kio/54298c16f37a81afb670f7b1c8fae323aedfb8a3