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Course contents

  • Getting Started with AI for VLSI Engineers5
  • Just Enough AI: Models and Context15
  • Prompting13
  • Trust and Verification14
    • Hallucination and Failure Modes
      • Invented APIs, Flags and Signal Names
      • Plausible but Wrong Numbers
      • Overconfident Tone and Sycophancy
      • Instruction Drift in Long Sessions
      • Subtle Bugs That Compile
    • Verifying AI Output
    • Evaluating AI Tools for Your Team
  • Using AI for Engineering Work17
  • Engineering Walkthroughs7
  • Practical Foundations for AI Workflows4
  • Engineering Data5
  • Retrieval-Augmented Generation6
  • Agents and Tool Use18
  • Agent Design8
  • MCP and Skills6
  • Multi-Agent Design6
  • Practical Capstone6
  • Tokens, Cost and Latency12
  • Agentic Workflows for Chip Teams18
  • Interview and Judgment10
  • AI Skills: Full Course Index
  • Working with AI Glossary
  • Working with AI Cheatsheet
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‹Hallucination and Failure Modes
  1. Topics
  2. AI Skills for VLSI Engineers
  3. Trust and Verification
  4. Hallucination and Failure Modes
  5. Plausible but Wrong Numbers

Verification and Safe Use · Beginner to Intermediate

Plausible but Wrong Numbers

Wrong numbers and stale facts in confident replies, with recomputation checks.

On this page

    Confident arithmetic

    Arithmetic in prose is generated, not computed. A multi-step total can be wrong in a middle step while the final line stays confident.

    The calc block below recomputes a five-step chain exactly. Each power is shown, so any line can be checked by hand.

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    chain success, per-step 0.9 over 5 steps (stated example values)
    0.9^2 = 0.81
    0.9^3 = 0.81 x 0.9 = 0.729
    0.9^4 = 0.729 x 0.9 = 0.6561
    0.9^5 = 0.6561 x 0.9 = 0.59049
    cost: assume 1 million input tokens cost 3 units (labelled EXAMPLE parameter)
    cost = 250000 / 1000000 x 3 = 0.25 x 3 = 0.75 units
    token budget split: 50% + 30% + 20% = 100%

    Outdated information

    Training data ends at some point, so prices, tool options and standard revisions go stale. A reply may quote last cycle facts in this cycle tone.

    Stale quoting

    Why it looks right

    What replaces it

    A price from an old table

    Exact figures carry authority

    A repricing under the current stated rate.

    A removed tool option

    It existed when the data was written

    The current option list from the tool reference.

    A superseded standard clause

    Clause numbers change slowly

    The current revision of the standard.

    How to detect it

    Recompute every quoted total from its stated inputs. The program below rechecks the chain product and the cost figure from this lesson.

    example.py
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    import sys
    
    def main():
        p = 0.9
        got = 1.0
        for _ in range(5):
            got = got * p
        if abs(got - 0.59049) > 1e-12:
            print("FAIL: chain gave " + str(got))
            sys.exit(1)
        units_per_million = 3.0
        tokens = 250000
        cost = tokens / 1000000.0 * units_per_million
        if abs(cost - 0.75) > 1e-12:
            print("FAIL: cost gave " + str(cost))
            sys.exit(1)
        print("PASS: chain 0.59049 and cost 0.75 units confirmed")
    
    main()

    Five multiplications give 0.59049, and 250000 tokens at 3 units per million give 0.75 units. Both lines match the calc block above.

    Common mistakes

    Common pitfall

    Copying a quoted total into a budget without recomputing. One wrong total then spreads into every line below it.

    Interview

    Likely probes: the recomputation habit and stale-data handling.

    • Recompute chains term by term from stated inputs.

    • Reprice with the current table under a labelled example rate.

    • Quoted precision is style: check the arithmetic, not the decimals.

    Check yourself A reply quotes a five-step chain at 0.9 per step as 0.62 overall. What decides?

    Recomputation: 0.9^5 is 0.59049, so 0.62 is wrong. The recomputation decides, not the confident tone.

    Check yourself A cost quote uses an old price table. What is the fix?

    Reprice with the current table under the stated example rate. Old numbers stay stale no matter how neatly they are shown.

    Related topics

    • Cost arithmetic

    • Evaluating AI tools

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    See also

    • Instruction Drift in Long Sessions
    • Subtle Bugs That Compile