> I've had to GC more than one repo where someone accidentally added an innocuous binary file

My god, the things I've seen in repos. vim .swp files. Project documentation kept as Word documents and Excel spreadsheets. Stray core dumps and error logs in random subdirectories, dated to when the repo was still CVS. Binary snapshots of database tables. But the most impressive by far was a repo where someone had managed to commit and push the entirety of their My Documents folder, weighing in at 2.4GB.