Well this is mathematically impossible...

Not necessarily:

Votes: 12 and 75

Mean: 43.5

Median (upper median): 75

While the standard e.g. wolfram alpha definition (which isn't normative) of the median would be 43.5 as well, it is an accepted practice (in plenty of CS grad classes, at least) to have the median guaranteed to be an element of the sample, normally the upper median is then chosen simply as "median". Hence the wiki definition having the qualifier "usual".

In fact, I was surprised that the median is strictly speaking not guaranteed to be an element of the set, using the majority mathematical definition.

So, not so much an error as a lazy CS convention ...