The frontpage slider of the Elegance theme sets its width during the loading but if the screen size or orientation changes, then the slider could just get lost behind the edge of the screen or show two slides at the same time (if the screen changes to wider than it was at loading time).

Steps to reproduce:
1) Configure at least two slides for your Elegance theme
2) Open the front page of Moodle (that has Elegance theme selected as the default theme)
3) Change the orientation of your device or resize your browser window