I believe I've found an edge case referencing the recent (Dec 4) changes of instance_eval etc. compatibility with current MRI 1.8 behavior.
Ruby backtrace and context :
https://gist.github.com/1be3d65c1ff8c23ce964 ( from sinatra trunk )
C function call :
Version ruby 1.9.2dev (2009-12-03 trunk 25980 doesn't reproduce.