Back in 2008, I voted in the Dem primary.
Rush was promoting his "Operation Chaos", and I found the idea amusing enough that I changed my registration from R to D, and then voted for Hillary against Obama.
Each state conducts their primaries differently. Are you in New Hampshire?
In NH, if you were a democrat who wanted to vote in the republican primary, you had to change your registration about a month before the election and then you are now a republican voting in a republican primary.
So now, for the 101st time, democrats could not vote in the republican primary and republicans could not vote in the democrat. Independents can vote in either primary and from what I heard on the radio, they said most chose the democrat primary. I'm sure there were some that switched registrations on both sides of the aisle.
So in review, democrats could not vote in the republican primary and republican could not vote in the democrat primary. It is a very simple concept.