Using this feature entirely turned out to be a blunder.
I set prices to 0 for free items and visibility as private.
On an average I receive 20 orders per hour.
I set a free gift via coupon at night 3.00 am and slept. Woke up at 11am to see there were 0 new orders and 50 complains in customer care due to checkout error.
Maybe you should not have a feature thats broken in the plugin or at leakest explicitly notify people of the issue.
I incurred a loss of at least $5000 in sales + reputation loss due to this.