I accidentally aborted an stg goto operation (configured to gpg sign commits by default, fingers didn't anticipate the password prompt so I typed the wrong thing, causing the commit to fail, causing stg to fail), and subsequent stg operations failed with:

error: failed to lock file '/home/lars/projects/k8s-perf-tools/db-perf/.git/refs/stacks/main.lock' for writing: ; class=Os (2); code=Locked (-14)

Removing the lock file allowed things to continue, but this seems like a bug: stg should clean up lockfiles on exit, and even if it fails to clean up lock files, any locks it holds shouldn't persist when the process exits.