[AnimalMuppet]

If the time to give a more accurate estimate takes more time than the actual work, you aren't dealing with an estimate longer than three weeks. If the estimate is less than a day, it's not worth getting more precise.

To your first point: Yes, that happens sometimes. When it does, your estimates can be wrong. (Hey, they're estimates - they're not prophecies.) If that happens very often, though, you might add a fudge factor for "that kind of thing". Maybe something like "unknown surprises crop up most of the time, and when they do, they take about 20% of the effort, so we'll make our best estimate, then add 20%". That won't be perfect either - sometimes it will be 40%, and sometimes 0. But, you know, estimates...