Contradictions in CPC: What's the Real Deal?

Tum log mock tests dete ho, I'm at my shift, so bear with me. I'm stuck on this one part of the Indian Contract Act (Section 29) - it says, "A contract is not void merely by reason of the mistake of one who affirms it as to any receiving order or other material circumstances which can exist or be supposed to have existed." Translation, right? You can still claim a contract is valid even if one of you is like, 'yeh toh chupke chupake gaya'. But wait, isn't this exactly opposite of Section 18, which says a contract is void if either party was induced by a mistake?

Tum log kya kehenge, CPC pehle se hi chalke phaila hai, or we students are just stuck in the middle trying to make sense of it? Someone enlighten me!