Frustrating little quirk when building reports in SSRS 2008 using Visual Studio 2008. If you write a custom query against a parameter dataset, and then change a query that references the parameter, the custom query used by the parameter is overwritten and reset to the default.
Now, you could keep a copy somewhere and replace it after every update, or, alternatively, use the <rd:SuppressAutoUpdate> tag by editing the rdl file directly.
<Query> ... <rd:SuppressAutoUpdate>true</rd:SuppressAutoUpdate> </Query>
I was close to raising this as a bug, when I found someone had beaten me to it: