SharePoint concurrency issues (users accidentally checking out older versions of files)

Question

This is a strange issue that I'm wondering if others have encountered when editing documents in a SharePoint document library.

Basic problem is as follows:

1. User1 edits document X
2. User1 checks in document X (say this new version is version 12)
3. Days later, User2 checks out document X, but the document they begin editing is actually version 11.

At step 3, sometimes User2 has discovered that they are editing an old version of the document, discarded their checkout and attempted to get the latest one, but to no avail. Even going into the version history of the document and selecting version 12 will show them version 11. For User1 and other users this is not a problem (things are working as expected for them).

Workaround is to have User1 checkout document X, save it, check it back in again. At this point User2 will get version 13 with the changes in it from version 12.

At step 3 we tried having User2 clear out the browser cache, open a new browser and reattempt to get the latest version of the file. This did not work.

Any insight into this problem would be appreciated.