Contractually, you can make whatever agreements you want. I would consider someone offering a warrantee for bug free software to be an inexperienced amateur. I would consider someone expecting a warrantee for bug free software an unrealistic and thus bad client.

Now obviously, if you are making software for a medical device and you are willing to pay for what that entails, you can get very close to “probably won’t kill someone” but it still probably won’t be bug free.