How unusual is getting a Masters degree in a slightly different field than Bachelors?
This depends on where you are. In the US, this would generally not be too difficult, particularly if one uses elective courses at the bachelor's level to take some of the courses in the "new" field. 

Here in Germany, it would actually be almost impossible to switch as you've suggested. The reason for this is that the "prerequisite" for enrolling as master's student in a particular field is usually taken to be the bachelor's degree in the same field as taught at the university!  This is known as a "consecutive" degree requirement, and makes life very difficult for people coming from outside Germany.