Blue dot

You have to be really careful as a medieval decorator. In the Middle Ages the sequence of making a book was as follows: first the scribe copied the text, painstakingly, sweating over a single parchment page for up to half a day; and then the decorator would add color. This meant, of course, that the brush of the latter artist was hovering over a completed page. These images show what could go wrong if gravity got the upper hand in this balancing act: a drop of blue paint fell from the brush onto the parchment page. The decorator must not have seen it, because the thick drop was allowed to dry and travel through time: the 700-years-old human error is now a pretty sight. 

Pics (my own): Leiden, University Library, BPL 64 (13th century).