The message it provides is:
"Sorry, while you edited this page someone else did already save a changed version. Please go back to the previous screen and copy your changes to your computers clipboard to insert it again after you reload the edit screen."
mike
Mike,
Do you think it might be possible to prevent a user from editing a page that someone has already started editing, rather than tell them that all the work they have done was in vain? 
Practically speaking, can we maybe add a checked-out flag to every page? I think this would be extremely useful 
Markus
. One thing though - might that tie up pages for large chunks of time and kill motivation? Maybe if there were also a setting which could allow contributors to see exactly who had a given Edit screen open at any one time and an easy way to negotiate turns - possibly with a built-in chat space in each Edit screen... After all, face-to-face collaborations would instinctively use communication avoid this kind of guesswork - why should it be different with wiki? Meanwhile I've been encouraging contributors to busy wiki pages to create their changes in a text editor and copy them in, but there are a load of reasons why that isn't a good idea - the danger of overwriting other people's changes with a former version, for example, as well as the lack of confidence that creates, and the convolutedness of the process.
Mira
Thanks, Mira, for hinting at the timing issue 
I absolutely agree that checking out pages for hours, days, ... would definitely kill any motivation, and I think it would be awesome if someone implemented your suggestion to combine the wiki with a chat window -- sounds like a near-whiteboard/application-sharing module for Moodle
For the time being, I've got a maybe somewhat more modest proposal 
: Assuming that a checked-out flag was in place, how about limiting the time any given wiki page can be checked out? Of course, it would be useful if this were an option in the settings of each wiki activity, so that a teacher may decide on the appropriate time frame, depending on what the learners are supposed to accomplish through the wiki. And to make sure that one doesn't "miss" the check-in time, we could maybe have a timer similar to the one in the quiz module; and some kind of reminder before the time is up would be nice, too 
Oh well, my wish list is getting longer and longer...
Any thoughts?
Markus
I am probably one of those (fairly non-technical) users who needs to have their expectations modified - when I jot things down in my wish list I have no idea how difficult or easy they would be to achieve. I should learn PHP so I can put my money where my mouth is (not a natural born programmer though).
Your proposal about limiting time per users - I think it may well be useful to have that as a setting to turn on or off. But there's something in me which balks at the idea of a time restriction as a response to a high-traffic wiki. I wonder about the cramping of contributor's creative impulses, grand designs, inspiration &tc. I think I'd prefer to subdivide contributors into groups with their own wikis.
But thinking about it - as you say, it's all about the purpose of the wiki - is it about content or process? If process, then my idea would work - if content, yours is better. If both - hmm.
So, flexibility, then?
Mira