It must be for something else. There are a variety of rebates from both the federal and provincial governments.
For instance, one rebate that I found that was exactly $500 was for installing a solar hot water heater. There may be many other examples.
Rebates from governments come and go as they want to either incentivize citizens to do something, or compensate them in the short term to make a tax hike more palatable (such as with the HST.)
In any case, the HST transition cheques for a family are: three separate amounts paid over one year: $330, $335, then $335 again, as described at this other question. There is no $500 transition rebate amount.