Windows service accounts.
Recently(probably 2 moths ago, but still – recently) had to configure a bit of security for our test servers, visible to the web. Of course started with securing services under their own special accounts, limited to a very small amount of rights. Everything went on smoothly except one lil’ thing – it didn’t work:) All permissions were set correctly, all relationships assigned well, but services didn’t start. Resolution(found it in 2 hours) was fairly simple – while creating accounts for my services I forgot to check the “Password never expires” checkbox....