Happened to me a few years ago. UA SFO-LHR. 2 hr transit to LH FRA-Berlin. UA aircraft had a mechanical and we sat in the aircraft for nearly 2 hrs. My bags were checked all the way through to Berlin. F/A came on PA and advised all connections will be re-booked so I assumed all was ok. Got to LHR and went to LH counter and was told I missed my flight and to go to the UA desk to see if they could do anything about re-booking me. Of course UA told me their obligation was only to get me to LHR (which was correct) so I was SOL. Went back to LH counter and ended up having to pay $400 for a new ticket. Never again after did I plan a trip with anything less then 3 hrs connection time if I were to be on 2 tickets. There is always a risk irregardless of how many hours in between you give (flight cancellations etc..)

Not necessarily true. Checking a bag through is just an agreement between partner airlines or airlines that have made an agreement for such services (interlining). All it ensures is your bag being transferred to the corresponding flight(s) till your destination. Depending on the airline, some may allow check in all the way through and BP issued by first carrier but every flight I have done like this required me to check in with the connecting airline at the airport on arrival. This is def the case with UA connecting to SQ or LH (again this is for 2 separate tickets).