Occasionally, duodenum may be confused with inferior vena cava on subxiphoid scans. This typically happens when the duodenum is distended with fluid (appears as anechoic tubular structure) and the physician performing scan is expecting to see a big IVC because of the history of congestive heart failure or something else. Here is one such example – acquired from a dialysis patient admitted with diabetic ketoacidosis. The patient was ~6 kg above his dry weight and thus the operator was looking for a distended IVC. However, this is too big!  

The patient likely had ileus in the setting of DKA. In this image, you can appreciate the connection between the stomach (round structure) and the duodenum. Moreover, there is no blood flow on color Doppler. Also, correlate with the anatomy.

This clip and accompanying explanatory images show how we can find the IVC just by fanning the probe. As we talked before, aorta is another structure commonly mistaken for the IVC.

Here is a nice example shared by @IMPOCUSFocus, where you can see a feeding tube in the duodenum. When there is no feeding tube or when you fail to notice it, the echogenic bowel contents may be mistaken for a clot in transit. So, beware and always fan the probe and make sure you are looking at what you are supposed to.