Ok, here is what happened.
A long time ago Columbia entered a transaction that involve Matt Forte, who at the time belonged to Round Rock. That trade was never proposed.
Later, but also a long time ago, Round Rock traded Matt Forte to Corona.
Recently Columbia entered new transactions into the old trade. The old transactions involving Matt Forte were still there, even though Round Rock didn't own him any longer. I'm guessing that old transaction was still visible to Columbia and he just didn't think to remove them. But it's possible that the old transaction was invisible, in which case that would be a bug.
The software is designed to check that all players still belong the teams involved in the trade before consummating the trade.
So from a software point of view, I don't have evidence that this is a bug.
I have a giant headache now from squinting across 4 databases to understand what happened (no team names or player names were in the databases, just numbers, which had to cross referenced).