1/ They were heard saying goodbye to their friends.
2/ This chair also comes with three-year warranty, which covers full cost of replacement parts and labor in the unlikely event that you have any problems with your chair.
1/ They (A & B) said goodbye to their friends. C heard A & B saying goodbye to their friends.
They (A & B) were heard (by C) saying goodbye to their friends.
An example of usage would be in a police report on missing persons:
They were heard saying goodbye to their friends at 11pm. Their car was heard driving away, but they never arrived home.
2/ If you have a problem with this chair the warranty means that for 3 years the manufacturer will replace parts for free and pay for labor - the cost of a repairman to fix it .
This is a very good chair so problems are unlikely to happen (not expected).
Other usage: In the unlikely event of rain, the band will perform in the hall instead of the park. (Rain is not expected, it is most unlikely)