I've had know-it-all customers interrupt me before and it's never really productive.
For example, I had a novice asking me questions about betta care. I was really trying to emphasize that:
1. A betta does best in a heated, filtered tank
2. that the larger tank they got the happier the betta would be
3. and that maintenance would be easier and less frequent in a larger tank
Because, of course, they were under the false impression that a bowl would be a suitable home.
As I was giving my "it's really easy" speech about water conditioner, the novice--head in the clouds, gerbils running--interrupted, "So if I got a bigger tank could I keep two of them?" and I told them, "No, bettas are very territorial and while they could live peacefully with most other tropical fish, they can't share with another betta." At this point a customer who was standing 20 feet away down the aisle interrupted, " WEELL, NO! you can keep lots of bettas together when they're female!" Then, he set whatever he was looking at down on the shelf rather firmly, turned around in a huff and walked directly out the door.
I looked back at the novice customer, and probably blushed a little; I could tell they were probably already a little too overloaded with information, and that taking care of a betta was going to be a more expensive investment than they realized they were getting into. So, I explained that we didn't sell female bettas, and while the interrupting guy was right, betta sororities don't always work because females can still be very temperamental and that it wasn't feat I'd really consider appropriate for someone just learning about tap water conditioner that day. Anyways, I talked to them a little longer and they didn't leave with anything. Another person not in the hobby.
It's really frustrating because at the store I'm at I probably have 100 conversations with people about dog food (that I really don't give a crap about) just to have that one conversation a week with someone who thinks they might want to join our hobby.
tl;dr I was trying to explain to a novice the basics not killing a betta and some guy who was looking for something to be offended by waited nearby and then ruined any trust I was building with the customer.