That first one - double the time - makes me laugh because of a true story.
I was responsible for determining how many servers would be required to consolidate a bunch of older, underutilized servers into a farm environment. I took a good look, and decided that we could do everything we needed to with existing servers.
Well, that wouldn't work, so I requested two new servers. At least we wouldn't have to worry about end of life of the existing boxes, and there was plenty of budget. Besides, you should always estimate on the high side, right?
I took that to my manager, who immediately doubled it. Of course, his manager and the PM agreed to double that.
So, we ordered up eight machines.
When the project was done, it turned out my original estimate - zero servers - was correct.
Of course, now that we had a farm of eight machines, we had to come up with a way to manage them. But that's a different project...