Americans gathered to remember and mourn the victims of the atrocity on Tuesday, with U.S. President Barack Obama paying tribute on the White House Lawn and thousands honouring a minute's silence in New York.
However, Modern Family star Hyland appeared oblivious to the potential offence she could cause when she took to her Twitter.com page and slammed United Airlines, branding staff "rude" and accusing them of "ruining" her day.
In a series of tweets, she wrote, "United hire people who are nice and helpful... Hate hate hate United. Rude and not willing to help one bit."
Hyland appeared to have a pang of conscience when the tweets disappeared moments later, but she then logged back onto the site to write, "Accidentally deleted tweets. Still infuriated with how my day has started and how United ruined it."
Oddly, earlier in the day Hyland was among the stars paying tribute to the 9/11 victims when she tweeted, "As a born and raised New Yorker from the lower east side, I will never forget what happened 11 years ago today. Remember 911."
United Airlines Flights 175 and 93 were two of four planes hijacked by terrorists on 11 September, 2011.
Flight 175 crashed into the South Tower of the World Trade Center in New York City, and Flight 93 was brought down over a field in Shanksville, Pennsylvania after passengers stormed the cockpit. Almost 3,000 people were killed in the atrocity.