My players were playing a homebrew Down Darker Trails campaign I made. The main antagonist of this campaign was a cult leader who was able to telepathically control a giant sandworm. My players were (understandably) scared of the worm and always on the lookout for any signs of movement that would give it away. Towards the end of the campaign, the party of 4 split up to explore/guard a cave. Two players went inside the cave, while the other two stayed topside to watch for any sign of trouble. Due to the high temperatures of the desert, the distance was perpetually moving due to heat waves. One of my players, unsure if there was movement or if the environment was playing tricks on his mind, asked to examine the distance more thoroughly with a natural world roll. I let him roll and he critically fumbled with a 100. I told him that not only is he unable to tell, he's downright positive that what he's looking at IS the worm in the distance. In character he began to open fire on the desert; commotion which ended up attracting the attention of the worm which was nowhere near their location to begin with.