could be hardware, software, design, past end-of-life, etc., etc.

better to fill unused executable memory with SJMP $.

i seem to remember ST docs even recommend fill memory with a trap pattern than is less likely to fail to trap even if the jump into an unexpected address could be in the middle of an instruction.

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http://en.wikipedia.org/wiki/Soft...ing_topics

Testing cannot establish that a product functions properly under all conditions but can only establish that it does not function properly under specific conditions.[12]

12. Kaner, Cem; Falk, Jack and Nguyen, Hung Quoc (1999). Testing Computer Software, 2nd Ed.. New York, et al: John Wiley and Sons, Inc.. pp. 480 pages. ISBN 0-471-35846-0.