United Utilities has contacted customers in Blackpool to say it has had reports of tap water looking "cloudy or milky".
The water firm said the water is perfectly safe and caused by tiny bubbles of air.
But United Utilities has since apologised on social media after inadvertently sending the messages to customers outside the affected area.
The initial message said: "We've received a number of calls from customers advising that your drinking water may appear cloudy or milky.
"This is nothing to worry about and is due to lots of tiny air bubbles in the water.
"To test this, fill a glass with water and let it stand for a few minutes. The water will begin to clear from the bottom."
The issue should improve in the next few days, it added.
But in a tweet on Saturday (August 3), United Utilities said some customers had received the message in error.
It said: "It has come to light that some customers have received a message from us advising of a potential white/cloudy colour to the water supply.
"This message was intended for our customers in the Blackpool area and we're sorry if this error has caused you concern."