It's certainly well worth mentioning. I know somebody who recently came home to find the house full of smoke. The daughter's magnifying make-up mirror had focussed the sun's rays onto a small spot on the wooden window frame and actually set light to it! It was a few minutes away from setting light to the curtains - and then....... As it was, it just burned a hole almost through to the outside and just needed filling when it had cooled down.
Some of these magnifying lights have a hinged shield above the lens, which will prevent this, and keep the lens fairly dust-free, too.
..it's simple enough to make something, if yours hasn't..
This, rather larger, concave mirror has caused some problems, too - http://www.bbc.co.uk/news/uk-england-london-23930675 .
When I lived in Cyprus, these solar cigarette lighters were a popular gimmick.