# Bug Discovery in Codercise I.11.4 of the Codebook

**URL:** <https://discuss.pennylane.ai/t/bug-discovery-in-codercise-i-11-4-of-the-codebook/4379>\
**Category:** Codebook\
**Created:** [April 3, 2024, 5:04am UTC](https://discuss.pennylane.ai/t/bug-discovery-in-codercise-i-11-4-of-the-codebook/4379 "2024-04-03T05:04:22Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![JH\_S](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/jh_s/32/2461_2.png) [@JH\_S](https://discuss.pennylane.ai/u/JH_S)\
**Post date:** [April 3, 2024, 5:04am UTC](https://discuss.pennylane.ai/t/bug-discovery-in-codercise-i-11-4-of-the-codebook/4379/1 "2024-04-03T05:04:22Z")

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By mistake, I used the PauliX gate instead of the PauliZ in circuit 2, resulting in an incorrect graph. However, the Codebook’s answer still indicates “correct”.

 ![image](https://canada1.discourse-cdn.com/flex012/uploads/pennylane/original/2X/e/e2449f60c7f3f7b52c7a6aa7f99690c33219c45f.png)

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**Author:** ![Alvaro\_Ballon](https://yyz2.discourse-cdn.com/flex012/user_avatar/discuss.pennylane.ai/alvaro_ballon/32/874_2.png) [@Alvaro\_Ballon](https://discuss.pennylane.ai/u/Alvaro_Ballon)\
**Post date:** [April 3, 2024, 6:42pm UTC](https://discuss.pennylane.ai/t/bug-discovery-in-codercise-i-11-4-of-the-codebook/4379/2 "2024-04-03T18:42:50Z")

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Thank you @JH_S for spotting this!

Seems like we have a test case to fix here 😅

Cheers,

Alvaro