1. Go to a project
2. Select Manage > Edit Picture
3. Under "Edit Project Picture > Custom", select "Choose Icon and Color..."
4. As the last listed icon, see an empty misaligned square.

https://we.phorge.it/source/phorge/browse/master/src/applications/files/builtin/PhabricatorFilesComposeIconBuiltinFile.php$52 calls listDirectory() (which returns a "list of files and directories") to get all icons under /resources/builtin/projects/.
However, https://we.phorge.it/source/phorge/browse/master/resources/builtin/projects/ includes a directory called v3 which gets incorrectly rendered as the last item.