Mylar balloons have grown in popularity over the years and while there is nothing wrong with that, when they aren't disposed of properly they can be trouble for power lines. Mylar balloons have a metallic coating that conducts electricity. Thus when one of these balloons comes into contact with a live power line, it can take out the power for the surrounding area and create a hazard for those called in to repair the outage. Brian Bretsch from Ameren Illinois lays out the stats on Mylar Balloon outages.

Proper disposal means deflating them after use, Some Mylar Balloons can actually be deflated and reinflated for reuse, but if you have no need to keep them, you should make sure they are deflated or popped and not allowed to simply float away.