Always be kind, and say it’s your fault.
If you don’t do it for the sake of the person you are asking for help, do it because it works better. That’s the most practical advice [0] ever given by Hans Rosling [1], the Fact master himself:
> In fact, I have the secret to how to get the best help immediately from any customer service, like the phone company or the bank or anything. I have the best line, it always works. You want to know what it is? When I call, I say, “Hello. I am Hans Rosling and I have made a mistake.” People immediately want to help you when you put it this way. You get much more when you don’t offend people.
[0]: Unless you are in charge of a developing country’s budget and have to decide between education and healthcare.