The following sequence causes serious errors (you can no longer view posts in a discussion):

1. Enable automatic completion on discussions (only available on OU Moodle - but it may be possible to trigger the same bug some other way)
2. Post a new discussion A w/ attachment
3. Post another one B without attachment
4. Go back to A, click the MErge button, go to B, click Merge here

The merge fails with an error; additionally, if you now look at A, the view fails with an error about missing folder.