When you have 0 ammo for your fusion rifle, sliding over a special ammo brick with slideshot and then right after trying to shoot will make your fusion charge indefinitely. It'll play the charge-up animation as long as you hold the trigger but never shoots any bolts. I can post a video if needed, but it's really easy to replicate. It happens A LOT when using a fusion with slideshot and has in many many times caused me to lose fights.