Want to hear a dirty little secret? We don't know. We can't know. First of all: Garbage in, garbage out.The information you bring us is almost never complete. Oh, you think it is. You have your W-2 (in Canada, T-4), and your retirement plan contribution slips, and everything else you think you need. But you forgot that 1099 (T5/T3), and that other slip that "got stuck in the mail".Did you spend a few days working out-of-state? You probably need to file a different state's non-resident tax return. But it's not shown on your W-2, so we'll never know.Then there's that little job you did for someone who paid you in cash. You didn't keep a record of it, or "forgot".Did you fix your neighbor's water faucet, and in return she baked you cookies? That's income (yes, really).In Canada especially, there are tons of "boutique" tax credits for doing things the government wants you to - sending your kids to post-secondary school (T2202A), or hockey, or dance, or piano lessons. Did you buy a transit pass?Most people remember to claim the credit for an eco-friendly vehicle, but I've seen that missed, to0.Are you absolutely sure you got all your medical receipts? Do you know how much you were reimbursed by your insurance company?I could go on, but you get the idea.Second: We miss stuff, tooOops. Forgot to input that slip. (and look at that shoebox above)Thankfully, not too often. We have processes to do it right the first time, and to catch it when we miss it. We have a review process, but... we're human. The good thing is, when screw up, we don't leave a clamp or sponge inside you. Our errors are fixable, usually at very little cost, if we find it early enough.Third: It's complicatedTax law is complicated, and getting worse by the day.So lemme see. You have a US LLC, a Canadian ULC, your wife is a US citizen, you work in 3 countries, and you earn $40,000 in total. So you can only afford to pay us $12 to prepare your returns, but you want them to be perfect?Yes, I exaggerate, but sometimes you have to be "practical" about how deep you're going to go to get to the "right" answer.In my area - crossborder Canada-US tax - it's even more challenging, because we have to take two complicated, ugly systems, and try to marry them. The answers are often bad, even when they're right.We try to get it as right as we can, under the circumstances. I think we do an excellent job in our firm - better than most - because our reputation is very important. But I recognize the reality that's out there.