I want to know why I still see Session Replays with no errors when replaysSessionSampleRate is 0 and I only use error sampling.

Error-sampled replays can still appear without an error in the UI if the SDK sends both the error and the replay, but an inbound filter drops the error without dropping the replay. React hydration errors are a common example: they’re filtered by default for errors, but that filter doesn’t apply to replays.

Check the replay’s Tags for replayType:

To keep those errors filtered and prevent them from using replay quota, drop them on the client (for example, with beforeErrorSampling, ignoreErrors, or beforeSend) so the replay is never started.

For hydration errors, see Why aren't some hydration errors being dropped by the SDK?.