Step 1: Identifying the Error.
The sentence “The boy told a lie lest he should not be punished” uses the wrong conjunction “lest” in the context. The word "lest" is used to indicate a negative purpose, but it should be followed by the verb in the past tense or should use "should" in the correct form.
Step 2: Correcting the Sentence.
The corrected sentence is:
The boy told a lie so that he would not be punished.
Final Answer:
\[
\boxed{\text{The boy told a lie so that he would not be punished.}}
\]