Create a bad git repository

gitprob.sh

git init --bare gitprob
git clone gitprob gitprobwork

cd gitprobwork
echo "good" > aaaa
GOOD=$(git hash-object -w aaaa)
git add aaaa
git commit -m "1"

rm aaaa
echo "more good" > aaaa
git add aaaa
git commit -m "2"

git push

# hash bad contents
echo "bad" > badfile
BAD=$(git hash-object -w badfile)

cd ..

rm -f gitprob/objects/${GOOD:0:2}/${GOOD:2}
mv gitprobwork/.git/objects/${BAD:0:2}/${BAD:2} gitprob/objects/${GOOD:0:2}/${GOOD:2}