Ahhhh!! I'm so excited!!
I've been blowing up Cap1 customer service by phone repeatedly about a 30-day late. Same story every time, they say they can't do anything about it, despite reiterating how I've been such a great customer, blah blah. So I had already prepared a whole GW letter and was about to send it.
Everything I've read suggests that the basic online dispute system has little success for GW situations, but as I was reviewing my report on a dispute center, I figured it wouldn't hurt to try anyway since it only takes a minute to click through. All I did was select "Other" and type something like "Loyal customer with timely payments 2+ years. Please remove 30-day late from Sept. 2010." (Btw, I've submitted a dispute for this in the past to no avail.)
Just got the dispute results that it was removed!! Status now says "Open/Never late." 
Seriously great feeling. I think this even tops huge CLIs.
But now the GW letter I'd written will go to waste! I got feedback on another one that it was too long, so this may be as well, but I'll post it here in case others want to grab some ideas --
I have been a Capital One customer since May 2010 and have been consistently pleased with my experience working with you. I am writing to inquire whether you would be willing to make a “good will” adjustment to your reporting to the major credit bureaus, Experian, Equifax, and TransUnion. The credit bureaus list a 30-day late payment on the above referenced account in September 2010.
I sincerely apologize for this mistake. At the time, I was in the process of changing my primary bank account, and the missed payment resulted from an oversight; I neglected to switch the information for processing payments. I am extremely sorry for this irresponsible mistake, but I learned from this error and made it a priority to ensure that all payments would be made on time thereafter. In reviewing my account, you will see I have been an exceptional customer and have paid early or on time every month for over two years.
In May 2010, I opened this account as a “starter” credit line to allow me to build credit. Impressed with the convenience your company offers, I recommended Capital One to friends, colleagues, and my husband, who opened his first credit card with you as well.
Many thanks to the credit line you extended to me, I have established credit worthiness over the past 2.5 years and have outgrown my Platinum card. I now qualify for many cards with excellent rewards and higher credit limits, and among others, I am considering applying for CapitalOne’s VentureOne credit card to better suit my current needs.
As I have been pleased with Capital One’s convenience and customer service, I hope to continue to use credit lines from your company for many years to come. Given the competitive nature of today’s credit card industry, I know how important it is to maintain positive relationships with customers. I hope you will deeply consider my request and honor this adjustment in accordance with my loyalty to Capital One and my consistent on-time payments since the time of my error in September 2010.
Lol that is awesome! Congrats!
Starting Score: EQ: 383 2/2011Haha, I just noticed something weird about it though... The way they deleted the 30-day late from Sept. 2010 was by just chopping off a few months of my history...
So now it looks a little strange:
Date Opened:
05/2010
Reported Since:
12/2010
I hope "consistency of dates open vs. reported" isn't another crazy thing that contributes to FICO scores. 
You never know with the secrecy of the system. I wouldn't put anything passed the scoring model honestly.
Congrats!
July 2013 score: EQ FICO 819, TU08 778, EX "806 lender pull 07/26/2013
Goal Score: All Scores 760+, Newest goal 800+
Take the myFICO Fitness Challenge
Current scores after adding $81K in CLs and 2 new cars since July 2013
EQ:809 TU 777 EX 790 Now it's just garden time!
June 2017 update: All scores over 820, just pure gardening now.
Great letter ! Thank you for sharing
