A small long-lived slices keeping a original arrays alive implicitly. Oracle recently undid the same optimisation in Java's strings because of that exact issue (which obviously broke code the other way around, code relying on cheap substrings would now take O(n) instead of O(1)). Erlang developers are also warned early and often about that behaviour in binaries.
Still something that should be in bold text in Julia 0.5's readmes, though; slicing is cheap now, but it comes with this very important caveat!
Iterating over the columns of an in-memory matrix, for example, is the use case that this change is meant to speed up.
I am saying this change is not quite "just a matter of performance", that the documentation and changelogs should be very very clear about the behavioural changes and the possible issues with the new behaviour, and that there should be an easy way to "unshare" slices and force a copy.
Like advanced indexing (http://docs.scipy.org/doc/numpy/reference/arrays.indexing.ht...) in the Python world?