What was doing the stinging in Maze Runner?

Early in the film, they tell Thomas that their biggest rule is only Runners enter the maze, no one else.

The best way to get people new to an area to do what you tell them to do? Tell them there's something out there that will hurt them. Now, there is actually something out there that will kill you, but it's clearly shown to not take an immediate effect.

As such, Newt telling Thomas that, "No one has seen a Griever and lived to tell about it," is more than likely his attempt to persuade Thomas to stay within the Glade and to quell his curiosity, or at least give him something to be afraid of to the point of not wishing to pursue his curiosity further.

They weren't counting on Thomas' level of curiosity, since none of them knew what he was capable of due to their memories being wiped. When Thomas learned that the sting isn't a cause of immediate death, and that others HAVE seen Grievers and lived to tell about it, he decides to venture out into the maze himself.

They have seen them, for example from the window.
By that quote they meant that no one has spend the night with them outside the Glade, and then come back in the morning.

Newt broke out in a laugh, but quickly cut it off. “I like you, Greenie. Now shut it and let me show ya somethin’.” Newt stepped forward and dug his hands into the thick ivy, spreading several vines away from the wall to reveal a dust-frosted window, a square about two feet wide. It was dark at the moment, as if it had been painted black. [...]

“Out there’s the Maze,” Newt whispered, eyes wide as if in a trance. [...] And we want to show ya why it’s not to be messed with. Show ya why them buggin’ walls close shut every night. Show ya why you should never, never find your butt out there.” Newt stepped back, still holding on to the ivy vines. He gestured for Thomas to take his place and look through the window. Thomas did, leaning forward until his nose touched the cool surface of the glass. It took a second for his eyes to focus on the moving object on the other side, to look past the grime and dust and see what Newt wanted him to see. And when he did, he felt his breath catch in his throat, like an icy wind had blown down there and frozen the air solid.

A large, bulbous creature the size of a cow but with no distinct shape twisted and seethed along the ground in the corridor outside. It climbed the opposite wall, then leaped at the thick-glassed window with a loud thump. Thomas shrieked before he could stop himself, jerked away from the window—but the thing bounced backward, leaving the glass undamaged...

They get stung and then they are shoved out to the maze to die, without being able to tell anyone else exactely what happened (as they go a little crazy). So when they say that no-one has seen a griever and lived, that is 100% true. There is no evidence to suggest any of the runners have seen one (prior to Thomas) while out on their runs.

"Grievers come out in the night",

"No one has lived a night in the maze",

Yet people have been stung "in broad daylight" and the entire community banishes them, saying it's a ritual.

This means that people have been stung and have therefore seen Grievers and made it out of the maze, morning or night, before.

This is the plot hole.

The monsters patrolling the maze at night, the boys coming out delirious about being stung after the serum has taken action and after the changing, the venom from the monsters, it is all part of the story.

The creatures that were doing the stinging in the maze occurred at day AND night. Bio-Tech creatures, created by WICKED (World In Catastrophe Killzone Experiment Department) were used, they were made by as their scientific name 'Bio-Tech' suggests, part machine and part organism.

These creatures looked like a bulbous creature:

'It looked bulbous and as big as a cow'

as Thomas (who's real name is Stephen but manipulated to believe hs name was Thomas) describes in the book, and the fact that it was like blubber and fat all piled into one. To support what I am saying, this is the link of what they looked like:

https://www.google.com.au/search?q=Grievers&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjr9vCxlu7SAhXFNpQKHVniCpcQ_AUIBigB&biw=1056&bih=555#imgrc=jdNHl3vI6SRZqM:&spf=192.

People here are confirming that they only come out at night, but Gally was stung on the otherside of the scale. He broke the Glade rules by stepping out of the Glade at day into the maze just to get the feel of it and out of nowhere, a Griever comes out of nowhere and rolls all over him, he is found soon after.

The griever is known to come out at night but because boys have been stung in broad daylight suggests otherwise because Alby was stung the same way the Gally was, only not in the same spot and the Griever was laying around like a dead stuffed toy and motionless until touched by Alby, there the Griever rolled all over him and he was thus...stung, and because of this, they get shut out in the maze at night.

This is what has been doing the stinging in the maze at night AND day.

The grieved was doing the stinging in the film. If was almost like paralysis when if stung so ye character would struggle to get away and live to tell the story of seeing the griever and being stung. In that situation Thomas was smart and managed to kill the Greiver and help his friend.

In my understanding:

"No one has seen Griever and lived to tell about it."

simply mean that, the returned-runners have not seen the Griever until Thomas and Minho return. Those who have met the griever were dead.

The statement could be more clear if it was:

"No one lived to tell about a Griever."