Oscillation is usually the signature of two platforms importing each other's calendars in a loop, so that each export reflects a different moment of the loop. Without a guard every flip is a date change: job rescheduled, cleaner messaged, job rescheduled back, cleaner messaged again. Lemon's own history includes an SMS storm from exactly this.
The guard records each committed change in a database table shared by every server (booking_oscillation_events); a change that reverts or repeats the previous one within OSCILLATION_TTL_MS = 48h is marked as oscillation. The stored checkout still follows the feed so the record stays loosely in sync, but the job and the cleaner are left alone.