Trade Failed: You no longer have all the items you offered in the trade

[GovernmentSecrets.gov]

Last night I tried trading a Mesa Prime set to someone and the trade failed. After a small delay we were had the prompt to resume the trade, but we couldn't make the trade go through because I kept getting an error saying "Trade Failed: You no longer have all the items you offered in the trade" even though they were still in the offer tab. I relogged and tried trading again, but this time I couldn't even put the items in the trade tab due to the same error. After a few minutes passed the items vanished from my inventory and the other player confirmed that they had received the items. Ultimately, the trade appears to have went through although very messily. 

I don't know what caused this or how to reproduce it.