At least in the US, that is very common. Obviously, it's not as common as getting a Masters in the same area as your Bachelors, but it's still pretty run-of-the-mill. This is especially true when there's some sort of overlap in the two fields, as there is between Mechanical and Civil/Structural Engineering.
A lot of top schools accept applicants into engineering Masters programs who lack engineering undergraduate degrees and even discuss this on their websites (Berkeley, Purdue, etc; I know those particular links are not for civil engineering, but they should at least highlight that it's pretty common for students to tweak their interests between undergrad and grad school). A simple Google search will bring up even more examples.
There are even programs out there specifically designed for people to make these sorts of switches (and even bigger ones). Boston University's LEAP is one example where students can have majored in just about anything and convert to engineering at the Masters level.