Its not much fun being the one to tell folks that this or that particular identification is questionable, so perhaps this page will give you an advantage in avoiding some common pitfalls. This can lead one to think they see something that is not there. These visitors don't understand the variation in our resident species, and don't realize that Herring Gulls here occur only in specific locales.
Misidentified gulls are common on lists from pelagic trips as well.
These discussions are not meant to be comprehensive but may just be helpful.