Nadine Schellenberger of Zum güldenen Stern, a half-timbered inn and pub founded in the 16th century in the centre of the southwestern town of Ladenburg, said the royals stopped in while she was hosting a wedding party.
"I didn't recognise them. I mean without crowns and scepters ... I am just not up on royal families and I don't have time for glossy magazines," she said, of the incident which took place late last week.
"Whether you're a street sweeper or queen – we just didn't have a table or enough manpower in the kitchen at that time and our hands were tied."
Nevertheless Schellenberger and her husband Michael plan to send a letter to King Carl XVI Gustaf and German-born Queen Silvia to apologise.
"We want to tell them that we are sorry we didn't recognise them immediately and that we weren't able to serve them but that we would be delighted to welcome them another time," she said.
Press reports said the royal couple and their entourage managed to get a pizza on the market square.