Exchanging Coal for Iron Ore?
I have never noticed this before, but in a game I started a couple of days ago I noticed that Iron Ore would get "stuck" at a number (not a specific number) at times. It would "loosen up" if I saved and re-started. I also noticed that at times the amount of Coal would drop drastically. Not often though, but it threw everything out of order, so I got a little frustrated. When it happened this time I had just checked the amount of Iron Ore (which was stuck again), and noticed that it had increased by about (maybe exact, but not sure) the same amount of Coal that had gone "Missing". I try to keep all Minerals as close to 1000 as possible, that's why I noticed it.
I don't think those things could get accidentally renamed/swapped.
Things are taken out of the town's inventory when moved by vendors though, so if you have a bored vendor who decides to move things around, you could have some 100-units loads disappearing and reappearing. If several vendors do that for both ore and coal, you could get the illusion of a swap I suppose.
Another thing is the way mines work. The workers dump things just outside and then those piles of ores or iron are taken "at some point" by a laborer or miner leaving work or trader and brought in batch to storage where it finally gets counted. The production doesn't feel very linear because of that, it instead feels like it's happening by big chunks.