I am the gm for a 5E D&D game, just using the Mines of Phandelver as a base and adding my own things to it. The party had decided to visit neverwinter to get supplies and gather information on the next part of the quest and the halfling ranger of the group decided they wanted to learn how to rouge and was following some dwarvish theives into a sewer to ask for lessons. The rest of the party were in a pub across town and telling everyone about the dragon they had just killed and getting pissed.
ME - ‘Ok, so the air is filled with a thick cloying smell, you can barely breath in this place. Roll a constitution saving throw.’ /player passed/ ‘Right so you manage to not throw up with the scent of shit, though you are struggling to breath a bit, what do you want to do.’
Ranger - 'I wanna roll perception, see if i can trail these dwarves.’
Me - 'Right.’ /crit fail/ 'Ok, so you can see bugger all, its pitch black and the only source of light is coming through the open sewer cover above you.’
Ranger - 'Oh…can i try again of i get some more light?’
Me - 'Sure…go ahead.’
Ranger - 'Right so im gonna light one of the torches in my pack using my tinder box.’
Cue silence from the whole table as people stare at the ranger.
Me - 'Are…are you sure you want to do that?’
Ranger - 'Yea, how else am i going to see?’
And that is how the sewer system of neverwinter exploded, the halfling ranger was shot into the air, almost died, was thrown into the river and then in jail with three shit soaked and burnt dwarves while the rest of the party just laughed as the pub owner battled to keep a wave of sewage which had blown back up the holes in check and the local wizards had to band together to clean the streets with magic.