Hey Jason,
We received a 221g for not having an annotation on her marriage certificate. Once we received one from the PSA, we went over the LBC to ship the documents. Since it was the day after her interview, we were pressed for time and were not expecting the 221g. We sent the submission slip along with the required documents to the Embassy. Like many others, I did not know what the UID number was or where to look for it. So we left it blank. I wouldn't be surprised if we receive another 221g for not placing the UID number on the slip. It's extremely frustrating for something so minuscule.