To be honest, I'd say around 60% of the coasters in my count are like that. I often don't even recognise them. I'll remember one or two from each park, but that's usually about it. That said, possibly more in the spirit of the topic, some rides are genuinely very forgettable. One that springs to mind (for some reason - maybe it's not that forgettable) is Hoosier Hurricane at Indiana Beach. I know I've ridden it, I know I've taken some nice pictures of it, I know it's location is lovely, but I can't remember a damn thing about how it rode. Not a single thing.