I read somewhere that the stain and the sealer should be of the same base. Oil based or water based. They should not be mixed.
Is there truth to this? If so, what will happen?
Are both combinations bad or can you do oil on water or vise versa?
As with paint -- the best philosophy is to stick with oil-oil, water-water, or oil over water.
However, it's is not a 100% rule. It is possible to apply water-based sealer over oil-based stain IF, and only IF, the stain was given more than adequate time to dry (days, not hours). That said, it is very possible that you can still have issues with bleeding or peeling.
So, for my money, I'd rather not risk it just to save a few bucks.