See https://el.wikipedia.org/w/index.php?title=%CE%A7%CF%81%CE%AE%CF%83%CF%84%CE%B7%CF%82%3AElitre_%28WMF%29%2Fsandbox&diff=4757904&oldid=4757901 .
Replace one or more words with "Foobar" (that is, highlight the word(s), then type Foobar). Works once, works twice, then the third time corruption happens and le wild snowman appears.
Must say that I only managed to reproduce that in my sandbox, which features content from https://el.wikipedia.org/wiki/%CE%95%CF%80%CE%B9%CF%84%CF%8C%CE%BA%CE%B9%CE%BF , and in this article page.
It happens in Chrome and Firefox, tested under Windows 7, Ubuntu 13.10 and OS X 10.8.
During an attempt to reproduce, VE also suppressed existing nowiki tags: https://el.wikipedia.org/w/index.php?title=%CE%A7%CF%81%CE%AE%CF%83%CF%84%CE%B7%CF%82:P.a.a/%CF%80%CF%81%CF%8C%CF%87%CE%B5%CE%B9%CF%81%CE%BF&diff=next&oldid=4757859 .