Place a fuel source block high in the air so that it is flowing down to the ground, use a flint and steel to light the bottom of the fuel stream on fire. This happens

When a fuel flows on top of a block that is replaceable, it treats it as solid and flows outwards accordingly. So as fire spreads upwards the fuel above it treats it as solid and spreads outwards making a larger stream of fuel, which the fire also spreads to, causing a 'fuel explosion'.

In addition if fuel flows beside a block that is replaceable and drops an item (such as wheat, grass, redstone, etc.), it still drops the item but doesn't replace the block.

Update to latest forge and try again. Also, I wouldn't consider 'fuel explosion' a bug, but rather a bloody cool feature :)

Updated forge, works fine now.