Add a file to be committed:

    root@taz[sid]:/tmp/tg-bug> git add 1
    root@taz[sid]:/tmp/tg-bug› git st
    # On branch master
    # Changes to be committed:
    #   (use "git reset HEAD <file>..." to unstage)
    #
    #       new file:   1
    #

Commit works as expected:

    root@taz[sid]:/tmp/tg-bug› git commit -a -m .
    [master 8978e64] .
     1 files changed, 1 insertions(+), 0 deletions(-)
     create mode 100644 1

Now read tg help:

    root@taz[sid]:/tmp/tg-bug> tg help
    TopGit v0.8 - A different patch queue manager
    Usage: tg [-r REMOTE] (create|delete|depend|export|import|info|mail|patch|push|remote|summary|update|help) ...

tg has added it's own stuff to .git/hooks/pre-commit whilst running tg help.

Let's purge topgit to see it something breaks and add another file ...

    root@taz[sid]:/tmp/tg-bug> dpkg -P topgit
    ...
    root@taz[sid]:/tmp/tg-bug> echo 2 > 2
    root@taz[sid]:/tmp/tg-bug> git st
    # On branch master
    # Untracked files:
    #   (use "git add <file>..." to include in what will be committed)
    #
    #       2
    nothing added to commit but untracked files present (use "git add" to track)

... and try to commit:

    root@taz[sid]:/tmp/tg-bug> git commit -a -m .
    .git/hooks/pre-commit: 2: tg: not found
    exec: 2: /pre-commit: not found
    zsh: exit 1     git commit -a -m .

So tg breaks unrelated tools by adding a shell snippet without proper checking.