One of the more common problems I - and I'm sure most electricians - come across is loose or detached connections inside sockets, switches, etc, something which often isn't noticed until a light or socket fails.
A loose connection might not seem like a big deal but think about this; a poor connection means increased resistance. Increased resistance generates heat which causes deterioration or damage to the wiring, resulting in it failing or, in serious cases, causing a fire. I've seen more than one instance where the wiring inside a socket has been very badly charred.
Also, if the bad connection is in a ring main, that could mean more current flowing through the cables than they are rated to carry. That can result in an overload, another serious fire risk.
You might have a light which flickers when it's turned on or you may hear a faint crackling sound from inside a socket or switch, or possibly even see a spark; that's called arcing and is a good indicator that there's a loose connection.
As always, my advice is if you're in any doubt, consult a qualified electrician. Better safe than sorry.