Satellites - garmin may say it has satellites but it'll still take a bit of time to get it properly right. Buildings will also have a say. It should vaguely remember where you shut it down, but mine still has to think about picking them up outside my house (but is fine with getting them at some random moorland car-park miles away - maybe garmin also make GPS for doggers?!)
Segments - the 'auto-segment' feature is a pain in the arse and they should really get rid of it - I keep on having to delete segments I make during orienteering events, as they're never going to get run again (and they're also pitifully slow!). Also, calling the ascent of Sandy Heys 'A57 Climb' strikes me as bordering on sacrilege!