A bizarre campaign against grammatical incorrectness has landed two young Americans in deep trouble. The pair, Jeff Deck and Benjamin Herson, who have roamed across America using marker pens and Tipp-Ex to correct bad spelling and grammar on less-than-literate signs, went a little too far when they amended a historic, hand-painted noticeboard at Grand Canyon National Park. They were arrested, given probation, ordered to pay a $3,035 (£1,640) repair bill, and banned from all US national parks.

The two are the somewhat nit-picking brains behind the Typo Eradication Advancement League (Teal), a sort of provisional wing of the Plain English Campaign. In March, it launched an Outreach Mission to correct grocers' apostrophes and spelling faux pas – coast to coast. Armed with only Tipp-Ex, chalk, and permanent markers, Messrs Deck and Herson travelled nearly 12,000 miles in 73 days, and identified 423 instances of signage marred by mistakes in spelling, punctuation and grammar. They made 231 corrections.

In Texas a firm called Wings announced it was "Now acepting application". There was a sign for "Dillettante" chocolate; posters referring to "recepies"; "cake's" and "birthday candell's"; signs that had the word "priveleges"; and a lot of "your" rather than "you're". An eatery in New York offered "chicken parmasan" salad, and a grocery, traditionally the home of the misused apostrophe, which called itself a "grocerry". They found menu boards advertising "today special's" and "capacino". A sign that warned "pedestrians use walks not roads"; a T-shirt shop missing the dash between the "T" and the "shirt"; an Army-Navy store offering a "hellicopter" helmet and a bullet "bandoleer", and a "Sweedish" berry drink. Mr Deck identified the correct use of the apostrophe to be America's greatest grammatical blindspot.

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A star of local spelling bees as a child, Mr Deck set up Teal after attending his five-year reunion at Dartmouth College. He said: "I was speaking with some of my classmates who were becoming doctors and lawyers, and other people who could have an impact on the world and I started to wonder how I might be able to do that... fixing typos was what I came up with... I've always been aware of typos wherever I go [and] I figured that it was a national problem".

Soon Mr Deck had founded Teal, complete with a website, blog and a typo correction kit. The tour began on 5 March from Mr Deck's town of Somerville, Massachusetts, and led the pair through more than 20 states correcting public signs and "other venues where innocent eyes may be befouled by the vile stains on the delicate fabric of our language," according to the Teal website, jeffdeck.com. Asked if it had all been worthwhile, he said: "Certainly! There are a lot more people out there now carrying Sharpies [a brand of marker pen] around with them."

And Mr Deck and chums are collecting a fan base. One Ruth M Newton, for instance, wrote to the Teal blog: "Bless your heart for reassuring me that I am not the only spelling and punctuation nut left in North America."

At the Grand Canyon National Park, the men had found a 60-year-old sign with a misplaced apostrophe and a missing comma. They duly whipped out Tipp-Ex and pen, and made the corrections. They then spotted that "immense" was spelled as "emense". They were shocked, but stayed their hands. Mr Deck later wrote: "I was reluctant to disfigure the sign any further.... Still, I shall be haunted by that perversity, "emense" in my train-whistle-blighted dreams."

Despite their self-control, arrest and swift retribution followed.