One of the editors, a school teacher, replied saying that he hadn't added those characters manually. We still haven't found a way to reproduce this problem.

Which browsers and browser versions are involved?

More, from more users:

https://ca.wikipedia.org/w/index.php?title=Partit_dels_i_les_Comunistes_de_Catalunya&diff=prev&oldid=13430606

https://ca.wikipedia.org/w/index.php?title=Purita_Campos&diff=13441743&oldid=13383289

I will ask the user that replied which browser he is using.

I asked because I'm wondering if this is another case of broken browser plugins (bug 52327)...

The user hasn't answered yet. However, while bug 52327 is about "garbage" being injected in pages, that garbage consists at least in JavaScript chunks. In this case it is really meaningless garbage. Garbage so meaningless and relatively random that I still fail to deduce where does it come from.

Just in case those users all use Firefox, https://bugzilla.mozilla.org/show_bug.cgi?id=1015433 comes to my mind, though rather unlikely.
We really need more info on the browser(s) used....

This problem keeps happening regularly, e.g. https://ca.wikipedia.org/w/index.php?title=Ferran_Clavell&diff=prev&oldid=13600764

The most surreal edit we have seen so far: https://ca.wikipedia.org/w/index.php?title=Ana_Zelich&diffonly=1&diff=13511465 -- it includes ☃ and ♙ !

One user reported MacBook Pro amb 10.8.5 with Chrome v34.0.1847.131. He also said that he could reproduce it sometimes by typing "wrong" accents, e.g. accent + space.

This might be due to newbie users typing "L´aventura" instead of "L'aventura" (note the different ´ '). How to test this: at least in my International English keyboard you can type ´ by typing twice '.

(In reply to Quim Gil from comment #7)

it includes ☃ and ♙ !

These specific two icons (snowman ☃ and chess pawn ♙) are a debugging feature of VE and unrelated to the original issue reported here. ☃ and ♙ usually means that Parsoid has encountered something confusing in the wikitext markup.

See also Bug 63395