(Phil) Shouldn't be too bad. One is expected to get that note on the 'bone (not too often) but the worst thing about it is reading it, being all in bass clef. The rule is that if it's in the stratosphere and on a space it's a C, otherwise it's a B (usually Bb). If it's a D the composer/arranger is not a trombone player. These high notes are far better played by trumpets and cornets anyway. Power to your embouchure. :-)
(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...