If you use a confidence-interval estimates, you can choose how wide to make the confidence interval. I use 80% confidence intervals because that's what supported by the project management tool I use. 10% of the time, the project should take more than the high estimate (and 10%, less than the low estimate).
If you use a 99.9% confidence interval and miss the high estimate, then you're probably over-confident. (And your estimate was probably too wide to be meaningful.)
If you use a 99.9% confidence interval and miss the high estimate, then you're probably over-confident. (And your estimate was probably too wide to be meaningful.)