Temporary seki
The most likely way that a seki proves only temporary, is that one of the outside groups dies; with the effect that one of the groups in the seki lives at the expense of the other.
There is a theoretical possibility that one player in a seki gives it up (by a auto-atari), as a ko threat, which will then be a loss-making threat of quite serious proportions.
Example
This example of a temporary seki is taken from the solution page of RTG Problem 20.
Although the stones and the
stones seem to be engaged in a seki, the outside
stones are all dead, so effectively all the Black stones are dead.
To illustrate this, we see that White can play the moves to remove the outside
stones from the board.
Now it is obvious that the stones are dead as well, which means that the apparent seki was only temporarily.