Can happen if you move the control lever too fast.
I've always gone for the "engage neutral and wait until you know it is in neutral then engage fwd/rev"
Actually I always put both engines in neutral before any manueouvering or stopping including well before any lock lay-bys. Far enough so that if it all failed the boat would just glide in no worries. Just to make sure both gearboxes are working correctly.
You can't make thing happen fast with displacement motor boats anyway so there really is no point using the Morse control quickly.
On my narrow boat I used to have a rod and crank linkage to the gearbox but the same rule applied. Out of gear well before any manueouvering or stopping and when it's definitely ok carry on.
A few years ago I did have to tell someone in a Leeds and Liverpool barge in Molesey lock that he was still in ahead gear. He hit the upper gates quite hard. I could not understand why he thought it had gone into reverse. It was obvious it was still going forward.
It's the third time it has happened at the exact same spot in the last two years, I know the owners of two boats there that have been hit.
Barge/narrowboat turns around the end of the island and swings wide, it's pure incompetence/inexperience, no excuses.