Make sure the car has the latest ECU software installed.
There have been numerous revisions throughout the cars production.
Current version is AL I think.
Some cars have had a problem with the throttle cable vibrating, this just needs to be re-routed.
Dashboard lights can flicker, only solution seems to be to turn the brightness down a little bit, or if still under warranty get the dealer to fit a new dash but even then the problem can persist.
The bracket on the CAT can break.
Heatshields rattling. (all Focuses)
The little plastic clip that swaps the manifold over from short to long runners can break, requiring a new inlet manifold.
Check the wheel centre caps as the salt can get in the back of them and they go all scabby.
Front seats can creak. (all Focuses)
Passenger footwell can get wet when muppets don't reseal the outer cover over the pollen filter properly. (all Focuses)
Tailgate can rattle (all Focuses)
In general though they are reliable enough. Sister-in-law has had one trouble free for over a year now.