User 2 changes don't get saved yet.
Instead they get a message telling them about the contention but their text box still exists and they can still edit the content.
The make a few edits incorporating the changes already on the page and then hit save.
User 2's changes squash any change that User 1 made (hope User 2 got all of them from User 1).
User 2 is not cursing Kwiki because it now lets them submit their changes instead of simply displaying an error message and clearing out 3 hours of work they just did because User 1 made a quick change while User 2 was editing the stupid file!