environment
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emacs 23.0.60.2 (cvs'ed on Mar 18, GMT+8)
debian 4.0r3
gnome 2.14.3

description
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After emacs started, I changed to a new buffer.
Typed in 2 Chinese characters.
Those characters were invisible. I knew they existed only because the text cursor disappeared and bell didn't ring until C-b'ed three times.
I went to Options > Mule > Set Language Environmet > Chinese, and chose Chinese-GB.
Those 2 characters appeared.