Why should you not set your transmit frequency to be exactly at the edge of an amateur band or sub-band?

Transmitting exactly at the edge of a band or sub-band is dangerous because your signal is not a perfectly thin line; it has bandwidth. Modulation sidebands naturally extend above and below your center frequency, meaning parts of your signal could easily cross the legal boundary into out-of-band territory. Additionally, equipment is never perfect. Transmitter frequency drift caused by temperature changes or aging components, as well as minor calibration errors in your radio's display, can shift your actual transmit frequency. Combining all these factors makes it essential to stay well within the band limits to ensure you remain fully compliant.