Yes, meditation can lead to numerous negative effects, when practised in an improper way.
For example, if you do too much calming techniques (such as concentration on breathing, or chanting mantras), and if you forget to balance calming with awareness, you might fall in dull states. Thus you can get bad habit of being less aware, more passive and lazy, so your character will worsen.
In fact, some Christians were partly right when they criticized Eastern techniques (mantra chanting) for making people kind of addicts. It happens that after developing states of samadhi some people attach to such pleasant states, and their lifestyle and behavior start to resemble those of drug addicts.