Why two different types of app errors?

We have an application that is occasionally crashing in random places.
Some users get the typical fatal error:

"xxxxxxxx" has encountered a problem and needs to close. We are sorry
for the inconvenience.

But one of our users always gets something like this:

The instruction at "0x003de054" referenced memory at "0x00300034". The
memory could not be "written".

Any thoughts about why one user would get one whereas someone else would
get the other? Are there memory conditions or PC configuration issues
that would produce one error rather than the other? Thanks.