Introduction:
In the novel "The Witness for the Prosecution" by Agatha Christie, Mrs. Jordan describes a particular action as a 'fatal mistake'. The irony in her comment lies in the fact that she later makes a remark on Mrs. Slater's defense, which turns out to be a fatal mistake itself.

The 'Fatal Mistake':
Mrs. Jordan describes Leonard Vole's decision to let Romaine Heilger testify as a 'fatal mistake'. Vole believes that Romaine will provide an alibi for him, testifying that he was with her at the time of the murder. However, this decision ultimately leads to his downfall.

Irony in Mrs. Jordan's Comment:
The irony in Mrs. Jordan's comment lies in her later remark about Mrs. Slater's defense. Mrs. Jordan states, "I was against it from the beginning... I knew it would be a mistake." This comment is ironic because Mrs. Jordan herself made a similar mistake by underestimating the defense strategy.

Explanation:
- Mrs. Jordan's statement about Leonard Vole's decision to let Romaine testify as a 'fatal mistake' reveals her belief that it will lead to his conviction. She believes that Romaine, as a foreign woman, will not be a credible witness and that her testimony will ultimately backfire on Vole.
- However, the irony lies in the fact that Mrs. Jordan later makes a remark about Mrs. Slater's defense strategy, stating that she knew it would be a mistake from the beginning. This comment is ironic because Mrs. Jordan herself made a similar mistake in underestimating the defense's strategy.
- Mrs. Jordan's underestimation of Mrs. Slater's defense ultimately proves fatal for her case. Mrs. Slater's defense cunningly manipulates the evidence and witnesses, creating doubt in the minds of the jury. This ultimately leads to Leonard Vole's acquittal and Mrs. Jordan's defeat.
- The irony in Mrs. Jordan's comment highlights the fallibility of human judgment and the unpredictability of legal proceedings. It serves as a reminder that even those who consider themselves knowledgeable and experienced can make fatal mistakes and be proven wrong in the end.

Overall, Mrs. Jordan's description of Leonard Vole's decision as a 'fatal mistake' and her later remark on Mrs. Slater's defense strategy reveal the irony in her own underestimation of the defense. This irony emphasizes the unpredictability of legal proceedings and the fallibility of human judgment.