There are some known cases where a script that runs fine normally will generate a spurious error when you’re in debug mode, thus making it impossible to debug it.

If you run into this sort of situation, try changing any repeat with … in blocks to normal repeat with loops. For example, this script chokes in debug mode:

tell application "BBEdit"
    tell document 1
        set L to (get every word where its text begins with "t")
        repeat with aWord in L
            tell contents of aWord
                change case of it making capitalize words
            end tell
        end repeat
    end tell
end tell

But this version works fine:

tell application "BBEdit"
    tell document 1
        set L to (get every word where its text begins with "t")
        repeat with i from 1 to (count L)
            tell (item i of L)
                change case of it making capitalize words
            end tell
        end repeat
    end tell
end tell

Let us know if you run into any other cases.