I happen to know the Captain of one of the aircraft involved, who is a highly experienced ex-Air Force pilot. He told me how he inadvertently inserted the waypoints incorrectly and then failed to miss the error when checking the Lat/Longs on the way point list, basically he saw what he expected to see, something like, N53 00.0 W050 00.0 . Instead he got N53 30.0 W050 00.0
I could see if you're not expecting to see 1/2 degree waypoint it would be possible to overlook it.
The crew were cleared direct to the first waypoint coasting out from Canada, bypassing the "Gate" waypoint which meant that a GNE chart plot would not have clearly shown the error (or was required, since they were still in radar contact) . Luckily the Gander controller caught the error and it was corrected before reaching the first fix, he informed them they were the 7th crew in a week to make that error. No violation was filed, but they got to have "Tea and biscuits" with the CP and it resulted in a company wide memo being distributed.