Errors with no backtrace

[Richard Lynch <lynch@aristotle.ils.nwu.edu>]

Sometimes I get an error message, but am not thrown into a break loop.
What's more, these errors are almost always non-informative, e.g.
Error: Illegal point 955841654 detected.
While executing assoc.
Is there any way of getting anyu more info at this point?  There are only
a few million different calls to assoc that could have been the one referenced.
Why DO some errors not give a break loop?
On a related note, the stack seems to be extremely short sometimes.  There are
function calls and so forth that are skipped.  Is there any way to get more 
detail about the steps involved?  Some things can't easily be stepped through.
(e.g. mouse-clicks and keyboard input.)