Yuguiです。次のようにすると落ちます。IOErrorが期待されるのではないでしょ
うか?

$ ./ruby-trunk -ve 'IO.open(3, "w").puts'
ruby 1.9.0 (2008-09-30 revision 0) [i386-darwin9.5.0]
:2: [BUG] object allocation during garbage collection phase
ruby 1.9.0 (2008-09-30 revision 0) [i386-darwin9.5.0]

-- control frame ----------
c:0002 p:---- s:0004 b:0004 l:000003 d:000003 CFUNC  :(null)
c:0001 p:0000 s:0002 b:0002 l:000001 d:000001 TOP    :2
---------------------------
DBG> : ":0:in `(null)'"
-- backtrace of native function call (Use addr2line) --
0x10faa6
0x274bc
0x274fb
0x3c753
0x5d5bc
0xfd04d
0xfd52b
0x5e4ef
0x5e572
0x28a06
0x113d04
0x113d30
0x4259a
0x4439c
0x44629
0x39a13
0x2c968
0x2caf6
0x244f
0x23b6
-------------------------------------------------------


-- 
Yugui <yugui / yugui.jp>
http://yugui.jp