Never assume the person you’re talking to knows what they’re doing. Never assume they know as much as you. They won’t.
Never assume your instructions will be taken as you gave them.
When you say, “Please type ‘123.company.com’ into Firefox.” You assume they’ll type in just that. So you’re be thrown off when they get an unexpected result.
They’re typed “www.123.company.com”.
They’re typed “123.company.com” into a search field instead of the address bar.
They’ve not even typed it at all and instead typed something completely different.
Never assume your instructions will be taken as you gave them. Always be ready for anything the customer may do.