On rare occasions a jackpot will trigger that spews out around a dozen stacks of single eridium bars.
The damage dealt by the grenade is scaled to the level of loot in the machine. This is a completely normal grenade and will damage any player character in the blast radius, not just the Vault Hunter who actually used the slot machine. Or, rarely, purple weapon and sometimes the chance for character or vehicle skins.
First and second symbols different (regardless of 3rd): No reward.(For 'Pre-Sequel', subtitute 'Moonstones' for 'Eridium Bars'.) Borderlands 2 and the Pre-Sequel Known combinations and rewards