The party is currently exploring an old temple deep in the jungle, but due to traps and undead encounters, we decide to barricade ourselves in an old infirmary and rest for the night, after chucking out the remains of the undead who previously occupied it. We set up a watch, and partway through the rotation… 

DM: Okay, roll for perception. 

Goliath Monk, currently standing watch: … I rolled a twelve? 

DM: You hear a shuffling sound, and there are scratching sounds coming from the door. Something’s on the other side.

Monk: Aw man. Well, the door’s still holding, so I keep standing watch. 

The human fighter then takes their turn as lookout.

Dm: Roll perception again. 

Fighter: *rolls a three* 

Dm: You don’t notice a thing, maybe a few weird sounds here and there but nothing to worry about. There totally isn’t anything weird going on at the door or anything. 

Nervous laughter from all of us; finally my Dragonborn Paladin goes up to stand watch, but when I roll a higher perception check….

Dm: You see that the door has been almost completely demolished by undead. Only a few wood planks are actually keeping them from pouring in, and you’re not entirely sure how it went on for this long unnoticed.

Paladin: How did you miss an entire zombie hoard trying to break the door down??

Dm: Beats me. The sound alone is loud enough to, as one might say… wake the dead… *Cue groans of agony*