This is because the FTL may install an optimized replacement while the DFG has already been jettisoned - so then the baseline JIT's cti_optimize sees an on-going DFG compile to reoptimize the code while also seeing an FTL optimized replacement.
Created attachment 210786 [details] the patch
Comment on attachment 210786 [details] the patch r=mark
Landed in http://trac.webkit.org/changeset/155209