Yeah, it's possible to construct sentences that are ambiguous without punctuation, but it almost always trivially easy to construct sentences that convey exactly the same meaning but are not ambiguous. Punctuation can transform an ambiguous sentence into an unambiguous sentence, but it's generally not necessary to construct ambiguous sentences in the first place.
For example:
Trivially easy to avoid. Simply replace the first sentence with or "William called me an idiot..." or even "William said that I'm an idiot...".
For example:
- William said I'm an idiot for turning down a date with your brother.
- William said, "I'm an idiot for turning down a date with your brother."
Trivially easy to avoid. Simply replace the first sentence with or "William called me an idiot..." or even "William said that I'm an idiot...".