Normally people are well intentioned, but sometimes you stumble upon repos with very strange licenses (for example, a javascript repo that has operating system restrictions https://github.com/stephen-hardy/xlsx.js/issues/8 ).
It's usually not of a concern for most people working on small projects (after all, another party has to notice and then decide to take action), but if you are trying to enter an industry with a highly litigious incumbent then you should make sure your ducks are in a row first.
If you count "not being allowed to use $LIBRARY because your company's lawyers won't let you use improperly code", then this is incredibly common.
The alternative scenario - using code released under an ambiguous license and then later getting sued for it - is much less common with large companies simply because good legal teams won't let that happen (see the above scenario).
I'm sure there have been examples of it, though - I know I've heard of those stories myself, even; I just can't think of them at the moment.
Unfortunately, it seems that most FOSS code on Github is not actually properly licensed: http://www.theregister.co.uk/2013/04/18/github_licensing_stu...