Recently my Illustrator CS6 on 10.11.5 El Capitan has started to change the stroke weight I'd like to a random anomaly. For example if I try set the stroke weight to 1mm it automatically changes it to 1.0583 mm. Again if I try another weight such as 0.5mm it then changes it automatically to 0.3528 mm. Has anyone experienced this before? I can't seem to see any logic in it and therefore wonder if it's a bug? Thanks!
Check whether you have any settings such as "Snap to Grid" or "Snap to Point" turned on.
Also check your document settings -- what units are you using? If it's in points, picas, inches, etc. that might affect your measurements -- change the document to millimeters and see if it keeps happening.
Smart guides can affect precision.
I've had some similar issues with Microsoft Word lately. I'll tell it to put 4pt after a paragraph and suddenly it's out to some number of decimal places.
If it's a bug, and it's showing up in multiple applications, it would indicate that there's been a change in the system's math processor. Another possibility is that, since Adobe Illustrator CS6 wasn't built for El Capitan, that there's some minor incompatibility with the way the 2 talk to each other.