Pokégear instruction booklet trap
The Pokégear instruction booklet trap is a trap in Pokémon Gold, Silver and Crystal that prevents the player from using Mom to enable Daylight Saving Time (puts the clock forward one hour) when it is between 11:00 PM and 11:59 PM, or disable Daylight Saving Time (puts the clock back one hour) when it is between 00:00 PM and 00:59 AM.
When the player tries to do this, their mom asks, "Do you want to adjust your clock for Daylight Saving Time?", instead of her 'normal' messages. Normal messages include "Is Daylight Saving Time over?" and "Do you want to switch to Daylight Saving Time?".
If the player says yes, their mom replies, "I lost the instruction booklet for the POKéGEAR. Come back in a while. Just do what you can.", barring the player from toggling it.
This trap probably exists to stop the player from changing the date forward or backward via Mom.
It does not exist in the Japanese versions because they do not feature the daylight saving time feature.
Thanks Pokechu22 from the forums for these pictures.