Sometimes you do silly things.
Like check big binary files into source control. Ones that shouldn't be in source control.
This causes problems. Okay, well not really problems, but it has some effects.
Cloning the repository takes longer, of course, since there are big files, and other operations can take longer too.
But fret not! If you need to purge files from Git, you can. Now, you can't just remove the file, since it's still technically in there, and because git deals with directories (and basically the repository) as a whole, instead of single files like CVS, SVN, and Perforce, there isn't an obliterate command. There is filter-branch though.
Doing this will alter the git repository in an irreversible manner. Backup first, and procede with caution. Remember, this blog post comes with no warranty!
This blog post was quite useful in helping remove some silly files.
It comes down to:
git filter-branch —index-filter 'git rm -r --cached --ignore-unmatch BIGTHING' HEAD
You can replace BIGTHING with a path to a file or folder, and that thing will be purged completely from the repository. This rewrites history all the way back, so doing this can cause problems when you push somewhere, or when people try to pull from you. They will have to clone a fresh copy and your push destination will probably have to be pushed from scratch too.