This happens from time to time. I've witnessed it on several major airlines as well as a few regional jets. Most airlines signal the 10,000 feet (sterile cockpit etc.) to the FA
's by using two bells. This is accomplished on most aircraft by switching the no-smoking sign off then back to on. At that time, most pilots then realize that they didn't even turn on the seatbelt sign prior to pushback and try to mask the chime by simultaneously switching the seatbelt sign to "on" when the no smoking sign is brought back to on during the 10,000 foot signal. I have to admit I've done this once when I was a CA
on the Brasilia years ago. We did accomplish the required checklists however I answered "on" to the FO's challenge of "Seatbelt sign" during the before start checklist without looking at the switch position. I called back to the FA
and she didn't even notice and said none of the pax did either.