A Certer underwater.
The Certers are a random event. Miles, Niles, or Giles present a player with a rotating image of an item, and three possible choices of what it is. If a player correctly identifies the item, they may receive coins, food (such as a kebab), gems, a spinach roll, or a half of a crystal key. An incorrect choice will result in the player's weapons and note-able items being noted and the player being teleported to a random location. The certers used to only note items; however, players took advantage of this by dropping any items they wished to remain un-noted and then answering incorrectly, resulting in the certers noting their raw materials and allowing the player to carry more. If you ignore them, they will give you 4 warnings and then teleport you somewhere and note your items.
Trivia
In RuneScape Classic, the Certers were employed in Draynor, converting fish (Niles), ore (Giles), or bars (Miles)