Yes, you can get fired for abusing your markout benefit. There is a record of when your partner number is used. Starbucks has done audits in the past.

You’re entitled to one free drink and food item on the days you don’t work. On the days you do work, you’re entitled to one free food item and as many free drinks as you want, no more than one at a time, for thirty minutes before, during, and thirty minutes after your shift at the store that you’re working at.

You can use your 30% discount as much as you want.

Your SM can run an audit too. And they do. and it’s a nonnegotiable termination.

As long as you dont continually abuse it you should be okay. When i was new i didnt realize we could only get 1 food markout a day so i got 2 or 3 every day! But as soon as i realized it was only 1 i stopped. You should be ok as long as it's only a one time thing and not reoccurring, i.e. not getting several markouts continuously.

If you're not doing this all the time and obviously abusing it, you'll be fine. I've definitely used my markout twice in a day by mistake before (sometimes baristas will just assume I'm using my markout and I won't know until they don't make me pay, and then it's too late). Those couple of instances have never come up or caused an issue.