Can I accidentally give up my right to sue for a contract breach in Ontario?

Yes. Rights under a contract can be waived — intentionally or inadvertently — through words, conduct, or a course of dealing that the other party reasonably relies upon. In Ontario, two main doctrines are relevant: waiver and promissory estoppel.

A waiver occurs when a party with knowledge of a breach (or a right that could be exercised) unequivocally communicates that they are giving it up. If you continue to accept performance under a contract after learning of a material breach, without objecting, a court may find you waived the right to terminate for that breach.

Promissory estoppel can arise when one party makes a clear promise or representation that they will not enforce a contractual right, the other party relies on that promise, and it would be unfair to allow strict enforcement afterward. Courts have used this doctrine to prevent parties from resiling from promises to extend payment deadlines, waive breach notices, or forego penalties.

The practical implication: if you know of a breach, do not simply continue accepting performance and saying nothing. Act promptly, document your concerns in writing, and state clearly that you are not waiving any rights. A "no waiver" clause in your contract is helpful but does not guarantee protection — courts look at overall conduct.