Don't worry, I knew all those things.
But
(1) Channel tunnel: sometimes people believe things as children and then just never reexamine those thoughts. Go to an aquarium and there's likely a 'fish tunnel' thing. A person hears 'channel tunnel', and thinks 'oh, it must be like that aquarium tunnel.' And never revisits the thought. I've worked in an engineering adjacent environment and grew up with news of them digging it so I knew it wasn't a clear tube, but others might not. I mean it's unbelievable that they've laid cable across the Atlantic. Many scientific achievements are unbelievable.
(2) Billericay and County Durham. They both 'sound' Irish. In England place names are not usually called 'county' whatever. In Ireland they are. An easy mistake to make. They are not places where famous battles took place and there's nothing really memorable about them so that people would have heard of them without a connection. They might have thought Durham and County Durham were two different places, like Gillingham.
(3) Terrorist groups as individuals. I can see how someone hears 'Al Quaeda' claimed responsibility, and think 'ok, that's the guy in charge.' It follows the naming convention Al [ Name ]. Not everyone is well educated or well read. They hear every now and again that 'a bad person has done something.' The Yorkshire ripper killed someone again, Sinn Fenn blew something up. Can't catch the bastards.
(4) Unicorns. Deer, Moose etc. Have horns. Narwhals have a single central horn. Lots of things are extinct. I can see how someone might believe they once existed in the way that dragons are analogous to dinosaurs.
I knew all those but who knows what 'unknown unknowns' are lurking?