Well, technically it's a problem with player's attention, but it's kinda frustrating.
Currently, when using trade machine, 2 players must select their pokemon, press "Ready" button and then press "Trade". The problem is, that "Trade" button doesn't require pressing by both players. So if youre lucky and fast, you can trade your trash pokemon for something valuable.
I will explain this glitch with issue happened on our server not so long ago. Player A wants to trade Solgaleo with player B for his Lunala. Also player A have, let's assume, have Magikarp in his party. So, they're opening Trade Machine UI and placing Solgaleo and Lunala. Then player A is starting to exit trade machine after player B pressed "Ready", like 'Enter-put in Solgaleo-press "Ready"-and press Esc' for some time, before player B can press "Trade". And in the last time player A places Solgaleo, player B is about to press "Ready", but if player A was fast enough, he can switch his Solgaleo to Magikarp and immediately press "Ready" and "Trade".
Yes, I understand, that is just psychology trick to distract second player.
Maybe it's possible to add mandatory confirmation for both players to press "Trade" button or some cooldown after placing pokemon in trade slot.
Sorry for long post and thank you.

Thank you for this report. We apologize for taking so long to get to this ticket. Due to the age of this ticket and that this is for a much older version I will be marking this ticket as expired, if this bug is still occurring in the latest version (8.1.2 as of writing this) then please make a new ticket.