If you do what I do and keep your Rails plugins in separate git repos using git submodules, you will encounter a git error if you've made changes to the local repositories for a plugin and try to deploy without pushing the changes up to the server:
fatal: reference is not a tree: 17f1db7f33c986f0d4a9dc0c5846322095cda96b
To resolve this,
cd into the relevant directory (vendor/plugins/whatever/) and push to the repo. It seems obvious, but I've made this mistake once or twice.