When making a one-off trip to Burlington on Friday I got distracted and forgot to tap-off after getting off the GO Train. I realized it immediately when I got on to a local Burlington bus at the station and the full fare was displayed there instead of what should have been a zero transfer.
There is a contact form you can fill-in on the GO website to report it. (Click-on PRESTO for the category, then "Forgot to tap off" for the sub-category)
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www.gotransit.com
Although it took until this morning, they surprisingly not only responded, but will apparently also issue a credit to the Presto card to make up for the difference of the "Fare Adjustment - Tap Off Not Detected" charge.