Always Scale Proportionally when Shift Constraint is held using the Scale tool
Holding SHIFT to constrain scaling proportionally should always constrain as long as shift is held.
Using the Scale tool (s) if you hold shift and scale, you need to stay close to a 45 degree path when dragging in order to keep the proportions locked. Illustrator EASILY unlocks the proportionate scaling when the users deviates a small amount from 45 degrees. It's very easy to do as a user and the amounts can be small depending on the degree the user moves. So it's very very easy to accidentally scale something to almost proportional size such as w:100% h: 92% while the SHIFT key is being held.
I wish holding SHIFT would simple lock proportions regardless of the angle I choose to drag. If I wanted freeform then I should be able to release SHIFT or not use at all.

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Thanks, Keith.
And here’s a link to that first report for those who is bothered by the behavior:
Holding Shift with Smart Guides enabled when using Scale tool is works unreliably
http://illustrator.uservoice.com/forums/601447/suggestions/36522916