HST repair flights
If sending a Shuttle to refurbish the space telescope is much riskier than going to the ISS (where it can stay for a while), then why not prepare another shuttle at the same time, just in case ? I mean by that: if once in space, it's deemed too dangerous to return, after inspection, then the next shuttle should be able to save the first crew.
Anyway, there are one or two missions left for that.
Raoul