You never know about OE trips for sure until you are released or check in. Once I was deadheading from DTW to NRT to give a Line Check the next day to SIN. I was sitting next to another DH pilot. Half way to NRT, he told me he was a reserve called out to cover the trip I was suppose to give the line check on as the CA had called in sick. Ended up deadheading back to DTW the next day.