Whatever you do twice will be perceived as a permanent policy decision. If you do something once that others have adopted as a permanent policy decision, you will likely be perceived as having adopted the same.
That is, expectations matter and create second-order effects, and previous actions set future expectations. This becomes self-reinforcing once people start acting upon those expectations and your choice becomes fulfilling those expectations or dealing with the many who feel as though they have been cozened, even if you explicitly told them not to have the expectation.
The ad hoc becomes an institution. You significantly changed some abilities, so you granted a free respec to everyone in that class. Do that a couple of times, and everyone comes to expect free respecs whenever any ability changes. Did you give one to everyone in the spirit of equality and generosity? Congratulations, your players now expect everyone to get one with every significant patch. They are going to build their characters around the expectation that they get the chance to reset every few months (if not more often). They probably will complain rather than quit if you do not give them the free candy they feel entitled to, but your game’s finances are probably even less threatened by the free respec. Expectations reinforced!
This fits in many places. Shopping falls immediately before a time when you expect a big sale. Children know the distance between what their parents explicitly forbid and what they will actually punish. My employer occasionally offers early retirement packages to encourage voluntary turnover of people at the top of the pay/benefits scale; the current round of retirees includes people who have put off retiring for years while they waited for the next incentive package.
Note the effect of having an incentive that displaces things in time without necessarily creating more of them. This is a known effect in political economy: if you set an incentive to do something, the majority of the money will probably go to people who were going to do that anyway. There is no reason to have a sale if you expect to sell out at full price.
: Zubon