A Google search for "macx_swapon FAILED - 12" shows that you're neither alone nor a member of a very big club.
I haven't got a clue what's up, but I guess it couldn't hurt to run a maintenance routine...DiskWarrior or TechTool Pro if you've got either, "Repair Disk" while booted into/from wherever you run it in Lion (your Recovery Partition?) if not. (Edit:
I guess it wouldn't hurt to repair permissions, too.)
For what it's worth, I don't find Safari 5.1.5 leaking memory, the component that used to be guilty now preferring to quit.Edit:
I just did a better search..."macx_swapon FAILED - 12 site:apple.com," and found both that you're less alone than I had thought and this
, which points to Parental Controls as a culprit. (The linked thread is the only one I found that reached any sort of conclusion.)