Last week was the second play session of our new Shadowrun (3rd edition) campaign. It is the initial stages of the plotline and for story purposes, and player buy-in, I need all my players to show up for the first few adventures. Last week I was down two players, one was having a baby and the other was visiting his father in the hospital. I still wanted to run something as a campaign can die if the "new campaign momentum" stops.
So, I ran a flashback adventure.
This was run as a singular run with only the table-present players participating. The characters of the missing players were "out drinking and couldn't be reached in time". The adventure was presented as something that happened early in the character's career, a few months before the start of the current campaign time frame. We were able to finish the adventure within the night so there was no carry over into next week's play. The characters were rewarded with xp and money...which they will keep when the game starts again in "current campaign time". It will be assumed they haven't had a chance to spend the money yet, and xp is a meta spend.
A group can run a flashback adventure and not have it impact too heavily on the "current time" adventures if they are run early enough in the campaign. However, there will come a point where explaining where the characters suddenly got a magic weapon or assault cannon becomes problematic. "Why didn't you use that in adventure X if you had if before adventure X?" At this point it may be too late to run a flashback adventure unless there is nothing the characters can bring forward to the current campaign other than information...and even that may need to be explained.
However, for this one time, it was a good way for my group to adventure within the current campaign even though we had missing players.