Can a buyer still sue if they knew about a problem before closing and closed anyway?

It depends entirely on what the purchase agreement says about it — this exact scenario is what "sandbagging" clauses are written to address, and Ontario purchase agreements land on both sides of the question. A "pro-sandbagging" clause expressly preserves the buyer's right to claim for a breach of a representation even if the buyer knew about the underlying problem before closing; an "anti-sandbagging" clause does the opposite, barring a claim on anything the buyer already knew about when it decided to close anyway.

Without either clause addressing it directly, the answer becomes genuinely uncertain and turns on general principles about reliance and waiver, which is exactly the kind of ambiguity a well-drafted agreement is meant to avoid. This is one of the more actively negotiated provisions in a purchase agreement precisely because it allocates real risk: buyers generally push for pro-sandbagging language to preserve every indemnity claim available to them, while sellers push for anti-sandbagging language to avoid paying for problems the buyer effectively priced in and closed on anyway.

Before closing with any known, unresolved issue, get the agreement's actual sandbagging position — express or silent — reviewed so you understand what closing anyway will cost you later.