Lew,
 They can bend the end to the point that they do not fit any longer.
 I found that holding the cassette with a pair of pliers and tapping
the spindel end on a table top will pop-off the end cap. Otherwise use
the church key and buy a whole lot of cassettes.

Draco

Getting even isn't good enough.

You might try using a film leader retriever and unload the film with the
spool still inside the cassette. This would only be a good solution if
your thumb problems are temporary and if you don't need to immediately
reload the cassette. Actually, if you need to immediately reload the
cassette, you could pick up some for free from your local 1-hour labs.
They should have a bit of film sticking out, to which you can tape the end
of your new film. Bulk load it and then use a can opener to get the spool
out when you're done. When your thumb heals you'll be able to go back to
your stored reusable cassettes.

hmm...those are reloadable cassettes we are talking about, not the modern
ones with smooth ends of the cassette body with the caps crimped onto these
ends?  With no channel at the ends of the cassette body into which the end
caps snap?   The usual way is to apply pressure with one thumb at the end of
the cassette body while pushing with the other thumb against the cap to free
it out of  that channel.   This approach does take a bit of doing with the
thumbs.

The alternative is to hold the cassette vertically over a hard surface with
the protruding spool hub downwards, then to strike the cassette sharply
against that surface.  The sharp blow of the spool hitting that surface will
force the upper part of the spool to free the upper end cap from the
cassette.  Try it.  In the dark, of course.  And if you have difficulty
holding the cassette while striking it against that surface, perhaps use of
a vise grip around the cassette body can be of help...added leverage and
less use of the thumb.

Yes, most cassette openers will bend the cap, especially a church key, just
strike the protruding end on the tabletop in the dark and the opposite end
will pop off.
If you hold your hand right the cap will not fly off into the dark nether
reaches of your lab.

Take a look around. I bought a load of plastic cassettes with a screw
top 20 years ago. I'm sure there must be someone, somewhere who still
makes these.

IMO screw caps are the way to go.
They are still being sold. Dan