A customer books a 3 pm slot on your website. They show up to a 3 pm appointment. Your rep is on another call, because the slot was also taken on the rep's Google Calendar by a meeting that was booked directly. Your website did not see the Google Calendar meeting. The integration is broken.
This is the oldest scheduling bug in the book and it still happens constantly. The cause is that most scheduling tools sync availability on a cadence. Every 5 minutes, every 15 minutes, every hour. In the window between syncs, both the website and the calendar think the slot is free. Two people book it. Collision.
The fix is two-way real-time sync. When the website takes a booking, it writes to the calendar in the same transaction, and the booking only completes if the write succeeds. When the calendar gains an event from any other source, it pushes an invalidation to the scheduler immediately. No batch jobs. No polling. Event-driven.
Most off-the-shelf tools still do polling. Some of the newer ones do push. The difference shows up in the bug count. Polling-based schedulers have double-booking as a known issue that happens occasionally. Push-based schedulers do not. When we build custom schedulers for clients who have had enough of double-bookings, we go push-based by default.
The second piece is rounding rules. If a meeting is scheduled in a rep's calendar for 2:47 pm to 3:33 pm, should the 3 pm slot on the website still be bookable? Different tools answer this differently. The right answer is almost always "no, block any slot that overlaps with the meeting, even partially." But many tools only block slots that start inside an existing meeting, and that produces ugly overlap situations.
The small details of availability logic are the difference between a scheduler that works and one that constantly has edge cases. It is not glamorous. It is also the thing that makes the whole product feel either reliable or flaky, and reliable wins.