Mission creep means “the expansion of a project or mission beyond its original goals.” source  It is a way to describe when something created for a good purpose somehow changes bit by bit into something it was not originally intended to be and not necessarily ‘good’.

An example is the law governing what can be done with CCTV cameras. The Regulation of Investigatory Powers Act [more info] allows certain public officials to use surveillance technology such as CCTV to watch the public. It was intended to prevent serious crime and terrorism. However, in 2009, a survey of local councils "uncovered the widespread use of covert surveillance to combat littering, fly-tipping and dog-fouling… …It is the use of anti-terror laws to combat this sort of 'environmental crime' that has infuriated the law's critics.” Find out more

In other words, a law intended to allow a small amount of public surveillance under strictly limited circumstances was ‘stretched’ by public officials to become something much more invasive than intended.