I have had this happen many times. If it is just a common for a set that is no where complete, I advise them of the error so that they can update their inventory (sometimes it has been a parallel issue and sometimes simply the wrong card). If I hear back from them acknowledging their error, I stress to them not to send it unless we have another trade; it simply is not worth the postage either side of the border. When it has been a parallel issue, I have pointed out the difference in sets as it is obvious that the other member was not aware of the parallel (who can keep up with these). Those members have usually thanked me for the info.
If it happens to be a card that I need to complete a set or a more valuable card, most likely the key card in the trade, then I will look to work something out with them, either another trade with the missing card included or that they send it along to correct the error.
This has almost always worked out. Most members will send the missing card either in another trade or by itself (even though I stress it is not necessary). A rare individual never acknowledged my message but he is a despical fraudster who many won't trade with because of what he did.
We all make mistakes; I have sent the wrong card at least twice that I remember. Things were resolved.
Bruno
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Check my Profile page to see my 2023 Goals and my Lists of sets near completion (5 cards or less) or sets getting close (less than 100 cards missing and 75% complete).
https://www.tcdb.com/Forum.cfm/Page/B/ID/0/?MODE=VIEW&ThreadID=25745&C=0