Problem discovered in tr (GNU coreutils) 8.13 on a Ubuntu 12.04.

Running those commands:

cd $HOME
echo "Linux" | tr [:upper:] [:lower:]

will result: linux

Running the same from /usr/bin:

cd /usr/bin
echo "Linux" | tr [:upper:] [:lower:]

will result: winux
which is obviously a bad answer.

In debug mode:

sh -xc "echo "Linux" | tr [:upper:] [:lower:]"
+ echo Linux
+ tr [:upper:] w
winux

Regards.