What are Access Violations and how do I debug them?

I've started doing some work converting a D1 app to D2 and I'm running into
dozens of access violations.  I thought D1 had cryptic error messages but
this is ridiculous!
As an example, I've got 2 TEdits that both use the same OnKeyDown code to
pop up a modal lookup window. When you select a record from the lookup
window it updates the Text property of the corresponding TEdit. The first
one works fine, but the second one gives me this stupid Access Violation
error with some dumbass address that I can't make heads or tails of.

WHAT'S UP WITH THIS?!

Any help would be appreciated, as I can't find anything in any of the help
files about it.