I developed a new report today on 10.2 that includes prompts alongside the results. When running these reports, the report appeared to execute twice. It ran once, looked like it was going to present the results, then it showed the hourglass one more time, ran some more, and then returned the result. I had never seen this type of behavious before in 10.1 and earlier.
I looked at the report spec and noticed that 10.2 is injecting a tag called useStyleVersion "10" in the XML. I removed this and the report ran normally again. Thought I'd pass this along in case others ran across the same issue.