Created attachment 125122 [details]
Happens only on 64-bit Release, almost consistently.
Created attachment 125123 [details]
Both tests included in test_expectations.txt file
I have rebaselined this two tests on r168097 <http://trac.webkit.org/changeset/168097> after the merge of the new multi-column mode.
Both tests pass now, so I think we can close this bug.