Crash IRB (interactive Ruby)
16 characters

String=0
String=0


Not the shortest, but I think it's funny that it doesn't crash until the second line. Generates roughly 20 lines of text before IRB exits. For some reason it cannot be shortened to for instance 2.times{String=0}.



edit

Of all the answers so far, this is the only one that has worked for me (and it works in all versions I could get my hands on), and I've tested all of them in these versions:

On whatever kind of Linux I get when ssh'ing into my university:

ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-linux]
ruby 1.8.5 (2006-08-25) [x86_64-linux]


Mac OS X Mavericks default:

ruby 2.0.0p247 (2013-06-27 revision 41674) [universal.x86_64-darwin13]


Installed through Homebrew on OS X:

ruby 1.9.3p448 (2013-06-27 revision 41675) [x86_64-darwin12.4.0]




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7 characters

Combining my first version (and/or @Howard's answer, for maximum cross reference) with @chinese perl goth's answer:

STDIN=0