Not applicable to a specific wiki – this occurred on the tools.wmflabs.org/iabot website after signing up for an account and then adding an email. A notice popped up, telling me:
Success: Your settings have been successfully saved, but your email needs to be confirmed before it can be used. Please check your email for more information. Please also trust the sender, IABot Mailer <email@example.com>, and check if the mail has landed in your spam folder.
Using a narrow window (half of a 1080p monitor, so a browser window about 960 pixels wide), the "IABot Management Interface" header (including navigational buttons) overlays most of this notice, so that only the latter part of the notification – "spam folder" – is visible.
A similar problem can be found on other pages, with the header overlaying page content. For example, when reporting this bug (before being redirected to Phabricator), the page title was overlayed so that the ascenders of the letters were covered (that is, in "Report a bug", anything above the top of the height of the 'a' was obscured by the header).
This is a visual issue which prevents parts of the page from being accessible to users using certain browser widths (most notably the flash notifications, though to a limited extent probably other parts).
Visual of the more minor impact (on the 'Report a bug' page):