Tolerance had to be doubled for boolean to work

What does this mistake mean? And why she sometimes is given?

Generally it means that your objects were close, but not close enough - just out of file tolerance distance. It means that the Boolean operation failed using the absolute file tolerance - so in this case Rhino tries loosening the file tolerance by half and tries again. If it succeeds on the second try, you will get the above message as a warning.

That being said, this is an indication of trouble, personally I would undo and go back and see why the objects won’t work within the normal file tolerance and fix them so they do. Otherwise, at best you will have some edge joins in the results that are out of tolerance; at worst, you will have objects that are problematic, like ending up with a rogue surface stuck inside (non-manifold object).

https://wiki.mcneel.com/rhino/booleanfaq
https://wiki.mcneel.com/rhino/faqtolerances

–Mitch

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