If its only just happened this once then I would assume you just didn't have it all the way in gear. Even if you see the little square around the R, it doesn't always mean it is fully engaged. I wouldn't get to worried about it right now. If it happens all the time then I would think something was wrong.
If there really is a problem though, I'm sure one of these tranny gurus will chime in and make me look the fool.
Take it easy,
Ron
FOR ALL YOUR WHEEL, TIRE, AND ACCESSORY NEEDS
CONTACT: EPLEY'S TOUCH OF CLASSIC 1-270-843-6892
OR TO GET ME PERSONALLY CALL: 1-615-943-6514
2000 Chevy Silverado 4.8L RCSB: smoked UPD, Flowmaster exhaust, De-screened MAF, Nelson 93 hot tune. Upcoming installs: trans-go shift kit, Eaton posi, billet servo, ported and polished throttle body...Near future: 6.0/4L80E install with 150 wet shot