There are many cases in the news of accidents with sometimes a large number of party balloons filled with hydrogen or other flammable gases.

One of the larger episodes was in 2012 in Armenia, where thousands of balloons exploded during a meeting, injuring 154 people, of which 4 seriously (the video is of poor quality): https://www.youtube.com/watch?v=jWEm2sS7Dw8

A smaller, more recent episode in India: https://www.youtube.com/watch?v=FH5JwHeKnZo

I had a science teacher that did this in class, then taped a match on the end of a yardstick and held it under the balloon. They made quite a bang. I wouldn't want to be right next to it when it went off.