How to leave a campaign gracefully:

Tell the GM and other players that you will be leaving, with whatever reason is the case, and ask about the possibility of returning later if events permit.

How to leave a campaign gracelessly:

Say that you can't make game because of other obligations, but that you would play as soon as those were taken care of. After two missed sessions, occupy the primary game area and invite someone who is intentionally excluded from (and not supposed to know about) the game, because your obligation didn't occur that week, starting a few hours before the normal game start time. Do not try to rejoin the game you claimed to want to play in!

"Accident" or "forgetting" don't honestly seem adequate to explaining this situation; it feels intentional.

If someone who prevented 6 people from gaming because he felt there was some reason ... had a reason? MAYBE STATE THAT REASON AND DISCUSS THINGS IN A CONSTRUCTIVE MANNER?

Damn it.