The house can afford to have a slot that has a temporary player's edge because the percentage of spins during which the odds might be tilted in the player's favor, is small compared to the percentage of spins in which the house has an edge. Here's an example that I got out of an actual slot machine PAR sheet:
A 5-reel, multi-line mechanical game has an overall payback of just over 90% and plays in two modes: normal mode and special mode. Based on the frequency of triggering the special mode and the special mode's average duration, it turns out that over the machine's lifetime, the game should be in normal mode during 65.48% of all paid spins and in special mode during 34.52% of all paid spins. In normal mode, the machine's overall payback is 58.84%, and in special mode, the payback shoots up to 149.19%.
Anyone who is fortunate enough to find this machine at their local casino, and wise enough to only play the game when it is in special mode, would enjoy a high player's edge. Do the math though, and you'll see that the overall return is just over 90% as I stated.
Another example: Currently, there is a popular video reel slot machine currently on casino floors all over the U.S. that is making casino operators a lot of money, in which temporary player's edges are frequently available. The manufacturer announced a technical bulletin and released a software update for this particular game because in its original release, servicing the machine and rebooting it would cause the game to boot into a state where all possible line and bet configurations (each one had its own separate board/reel state) had a temporary player's edge. Many players knew about this flaw and the advantage that could be gained, and nevertheless, the casinos still made lots of money even before the flaw got patched.