QUESTION:
Has the aperture door ever jammed?

ANSWER from Sylvia Baggett on April 10, 1996:
That's a great question and precisely the sort of thing we worry
about:  Should we do XXXX and what happens if XXXX breaks/jams/stops
working, what impact will it have on HST, the science instruments, the
mission?  In this case, if the door were ever to jam shut, none of the
science instruments would be able to take data.  For this reason, we
try to minimize the number of times that the door is shut; in fact,
it's only been shut a few times: when HST was first taken into orbit in
the shuttle payload bay, during on-orbit "safes", and during the
servicing mission when the astronauts installed new instruments into
HST while it was still in orbit.

When HST is around/in the shuttle, the door is kept shut to protect the
interior of the telescope, primarily from contamination from dirt, gases, 
fuel droplets, etc which could degrade the performance of HST once it's
released.  Once HST is alone in orbit and collecting data, if at any
time it becomes confused about where it is pointing, the computers have
been instructed to close the aperture door automatically, in order to
avoid any unexpected bright light getting into the telescope.  Some of
the instruments are quite sensitive and could be permanently damaged
if exposed to bright stars, moon, earth, and especially the sun.  The
risk associated with closing the aperture door is seen as less
important than the possibility of losing one or more of the instruments
or possibly even the telescope itself.  This procedure of having the
telescope protect itself automatically is called "safing"; that is, if
the HST computers detect a problem, they react immediately and
automatically to protect the telescope and instruments, to get them
into a "safe" state.  This "safe" configuration is maintained until HST
hears from the people and computers on the ground that it's ok to open
up again; this allows us time to investigate the problem and find a
solution.

There is an amusing anecdote about the first time the aperture door was
ever opened after HST was in orbit (of course it wasn't so amusing at
the time!).  HST had just been taken up into orbit, the shuttle
astronauts had tested some things out, released it, and backed away.
The door was opened, and worked just fine, but then immediately shut
itself!  As you can imagine, everyone was extremely worried, what on
earth had happened, what could be the problem?  After some
investigation, it was discovered that the door had been opened too
quickly, causing HST to shake, which caused the computers to close the
door!  The situation is similar to what you have probably experienced:
a door opens a lot easier than you expected, and it ends up banging
into the wall.  In the case of HST, there's no wall for the door to
slam into, just the telescope.  When the door opened so quickly, it
shook the telescope so much that HST thought it had a pointing problem
and "safed" itself, just as it has been programmed to do.  The solution
was to have the engineers instruct HST to open the door a little more
slowly; once this was done, there were no further problems with the
aperture door.