The phrase "first transgression" refers to the initial or earliest wrongdoing or mistake that someone commits. It implies that there may be subsequent misdeeds or rule violations, but the first transgression is what starts it all.
In some cases — for instance, if this is your first transgression — you might be able to get them to remove a late payment or two, if you agree to make those payments on the spot. (credible.com)
Some options seem to be: At the time of the first transgression, After a certain number of transgressions, When the dollar amount goes over a certain amount. (law.stackexchange.com)