(Darren) Although leger lines are part and parcel of trombone playing (especially 1st and 2nd trombones) I'm very glad they don't go into Tenor Clef which to me would make it unreadable instead of merely difficult. Treble Clef is never used because most 'bonists can't read that either unless like me they're also piano players.
[Raak] It happens often enough for you to realise that you shouldn't get caught out by it, if that makes any sense. Depends on what key you're in, you see...