the purpose of FMEA, or FMECA (failure mode, effects, criticality analysis) is to examine the hardware and software
for potential failures and their effect on operation and safety of the machine and humans.
the benefit is the discovery of degraded and failed modes of operation and safety
it is hard (impossible?) to design out all issues. the more different people look at the
machine, the more likely it is that a serious design flaw will be found.
That's why the designer(s) shouldn't do the reliability analysis or the FMECA.
basic modes of operation: normal, intermittent, incorrect, failed for anything
as you said, regarding I2C: total loss (like wires cut / or shorted / or inverted / wrong data rate at one end ),
intermittent (like bad connection, bad driver, bad receiver, multiple sources on at same time ... )
list all of the potential failures.
since there is a circuit involved, pins of an IC can be open, shorted to +VCC, shorted to GND,
they can oscillate (usually at the worst possible frequency)
if there is a transistor involved, each pin can be open or shorted to one of the other pins,
or it can oscillate, or anything else you can think of.
this should be useful
https://en.wikipedia.org/wiki/Failure_mode_and_effects_analysis
don't forget to review some of the works in the biblography