I thought it was Opera's fault,
but tested this again today and it's not related to the browser you're using.
It appears that when you edit as an unregistered user (on wikis which allow you to...) you'll always get an Edit conflict page when you try to save after you switched to wikitext.
Tested on multiple wikis on both ns0 and ns2.
Version: unspecified
Severity: normal
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=56767