An article today from The Points Guy "5 tips for booking flights to Europe this summer when the EU reopens to vaccinated Americans" includes #4: Keep an eye on your reservation for unexpected changes.
Back in December, in a spate of unbridled optimism, I re-booked my 2020 Italy trip for this September. I also used United Mileage Upgrade Awards to snag (relatively) dirt cheap business class from/to San Francisco (SFO)/Frankfurt (FRA) and wait-listed for 1st between Portland (PDX) and SFO. The legs from/to FRA and Italy are code share economy on Lufthansa, not upgradeable. The economy legs are short; it's the 11.5--12 hours over the US and Atlantic that I cared about.
About a month ago I decided on a whim to check my booking. My last outbound leg from FRA to Bologna had disappeared. No notice from United, and my booking on the web site wasn't even flagged for attention. It just wasn't there. Lufthansa had discontinued that particular departure time. After some quality time on the phone with a United agent I got re-booked on a later flight, but my 3-hour connection is now seven hours.
Then a couple weeks ago I got a email from United: Congratulations, you're now 1st class from PDX to SFO. Seat 1B. Yea!
Except they didn't tell me they moved my flight, and my comfortable 3 hour outbound connection is now 2 hours. I had originally booked with about 3 hour connections to allow lots of time for delays, extra Covid vaccination/testing hoops, etc. Two hours is the minimum I would like to see.