https://commons.wikimedia.org/wiki/File:WikipediaTomeRaider3.png
I cropped some nonfree software parts from this screenshot and uploaded the new version. When I hit the upload button, I was presented this, [[ https://commons.wikimedia.org/wiki/File:Fatal_exception_of_type_MWException.png | in red ]]:
[WzH-EQpAADsAABKNuaoAAADH] 2018-06-26 08:53:40: Fatal exception of type "MWException"
I just hit back and tried again. But now, the original revision is gone! It's still there, but the actual file now shows my new version.
Update:
Within a few minutes, all of Commons went down. "Our servers are currently under maintenance or experiencing a technical problem. Please try again in a few minutes." The old revision is still gone.
This was an effect of {T198350}, which was dues to erronous locking during page deletion, see T198350#4322869. While it's unavoidable that sometimes operations fail due to outages or bugs, the data loss described here should be avoided by improving the code that manages database updates during file uploads. See also @jcrespo's comment at T198350#4323879.