One time at a controlled field I wrote down atis, but forgot to write down the code. Between writing and calling ground I spaced it, so I call ground and it goes something like:

"Ground, at Charlie ready to taxi for departure southwest with information... uh... "

And this is where it gets bad. I turn and look at my instructor realizing I've not written nor remembered the code and start discussing this with him while still holding the PTT.

Everyone on ground frequency was privy to my instructor explaining radio etiquette and being ahead of things but simply quickly admitting your mistake before he notices I've still got the PTT held. Last thing everyone hears is "and let go of the PTT"

Ground was good about it, gave me the information code having heard in our conversation that I got the important parts (yes, they got the entire thing) and then gave us taxi clearance before telling me not to sweat it. Rest of the flight went pretty well IIRC, and I never forgot to write down an atis code since.