Is this informal acceptable?

[Howdydoodyfolks]

DD works in a place where every month they have a staff meeting.
Last week they bought up something the staff team were doing wrong on the tills, first time they knew what this procedure was. It was something,but not actually this, like before they were putting through red wine as Merlot when it was called another red wine.
This week she is called in by the supervisor and given an informal warning for doing this retrospectively! I find this out or order as she has been keeping to the rules now it has been explained.
What can she do? Suck it up or should she complain about it?
TIA

Was everyone else given an informal?

Im not sure RunningOutOfCharge - I am presuming so

Don't they have barcodes or is it a pub or restaurant, have they had training

No it isnt actually the wine scenario I was just using that as an example, it is something similar.
My question is really is a retrospect informal allowable - I have never heard of this happening before. Just wondered if anyone else had?

Is it reasonable that she should have known?
Was she the only one actually doing it and the meeting was just to remind everyone?
If everyone was doing it, did everyone get the warning or just her?

They couldn't give a warning until they were aware of it going on, so in that sense, it has to be retrospective.

Whether she should be the only receiving a warning or whether it should have been dealt with by making sure everyone was aware of how it should actually be done and saying any differences from that going forward would get a warning, that's a different question from the timing.

Thanks for your comments x