[generuso]

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

[trhway]

a party balloon - say a cubic foot - is about 2g of hydrogen. Involves 16g of oxygen. So we're talking 18g of very fast burning, borderline detonating mass. Releases 240 KJ of energy. To compare the hand grenade - 60g TNT https://en.wikipedia.org/wiki/F-1_grenade_(Russia) - releases the same 240 KJ of energy.