The biggest mistake is making it too complex. Keep the threshold low. Preferably just scanning a QR code. There are more pitfalls. Test your set-up beforehand to prevent glitches. Too much competition backfires. Don't use gamification as filler, but link it to the content. And make sure rewards are proportionate to the effort.
Know the risks and avoid them.
More on this in our article on gamification at corporate events.