There was a previous post on this topic (called Losing Data), and I tried commenting on there first. However I don't think my comment was picked up by anyone as there is no obvious way to show all new comments in the forum instead of just new posts.
Anyway I have come across a losing data scenario a few times in the short time I have been using Smartsheet to track projects across about 30 users. Basically I have discovered that if a user leaves the sheet open whilst someone else is updating it and then returns to the sheet making any change and subsequently saving (auto or manually) this will overwrite the changes made in between by someone else.
The individual cell overwrites are tracked so you can verify it has actually happened, but obviously rolling back individual changes is not really a feasible operation and since Smartsheet at this point in time lacks versioning on a sheet-level you are effectively stuck (read screwed).
In my opinion this is a fairly major flaw in the app which in a high-change collaboration environment could have massive impact. As it stands the only effective way to counteract the problem seems to be not leaving the application open if you are not using it, but this is definitely not a bulletproof solution and if someone forgets to close the app the scenario could happen.
Introducing sheet versioning would allow damage limitation, whilst forcing a refresh of a sheet before new changes are saved is more a risk avoider (the question is how would you merge changes from multiple parties especially if there is conflict)...