Disciplinary hearing for gross misconduct? What shall I do? (UK)?

So last week I had to have a meeting with my manager because something "inappropriate" was written on a customers receipt. Where I work, the top of the receipt was hanging out of the printer and I wrote something drug related (was a song lyric) out of boredom. However, I forgot I wrote something on there... show more So last week I had to have a meeting with my manager because something "inappropriate" was written on a customers receipt. Where I work, the top of the receipt was hanging out of the printer and I wrote something drug related (was a song lyric) out of boredom. However, I forgot I wrote something on there and unwilling gave it to the customer. My manager labelled this "gross misconduct". He had the option to suspend me there and then, but he chose not to because of the 2 years I have been working there, I have had a perfect customer service record and had excellent relations with staff. He still said that I will need to attend a disciplinary and that it is VERY unlikely I ll get dismissed and told me not to worry because what I done is just silly and the fact that I have a perfect record with the company. So should I follow his advice and stay calm?
PS there are colleagues that I know that have had fights in the store and argued with managers and staff, yet they are still working there even after their disciplinary hearings. Some have worse records than me. Should this give me confidence?
Many thanks
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