Aspect ratio suddenly changes

For no apparent reason, the aspect ratio of the image on my screen changed just now when I switched on my Mac Mini: the image is now squashed horizontally.

This happened once before - when I took the Mac Mini to an Apple Store Genius bar and they connected it to their own monitor.

I corrected it by fiddling with the settings but I'd like to know why it happened in the first place, why it's suddenly happened again and how I can cure it permanently. Any ideas, anyone?

Did you switch monitors? Different monitors have different sizes so it may look full on your original screen but may stretch on others.
What's the size of the desktop background in relation to your original screen?

Does it happen often? I wouldn't worry about it too much if I were you

Hi there.

No, it doesn't happen that often but it's really annoying when it does and if there was a way to fix it permanently that would be great.

If your external monitor is set it its native resolution from the Mini (System Preferences, Displays) and it suddenly changes aspect ratio (resolution size) then I would suspect a defective or intermittent cable. Check the connector from the Mini in addition to the connecting cable from the Mini to your monitor.

interfacelift — wallpapers, icons, themes, and iOS apps is a great website to get large HD desktop backgrounds at!

You can check your resolution here; What is my screen resolution and just download the corresponding picture size at interfacelift.com.