Sooo... you'll probably be able to tell by my crazy code that I've been round and round with this one, but all I want to do is assign a solution category based on the user's drop-down selection on the originating case. I ensure the dropdown selections always correspond with a solution category using a seperate process.

The dropdown field is "feature_area__c". 

My test code all passes fine, but when I execute this, I get the awesome "An error has occurred while processing your request."

public static void AssignSolutionCategory(solution[] solutions)
{
Map<Id,string> solutionCaseMap = New Map<Id,string>();
List<string> solutionIdList = New List<string>();
for (CaseSolution cs : [Select SolutionId,CaseId from casesolution where SolutionId in :solutions])
{

//I do this to ensure I only have one case match per solution
if (solutionCaseMap.containsKey(cs.SolutionId)==false)
{
solutionCaseMap.put(cs.SolutionId,cs.CaseId);
solutionIdList.add(cs.SolutionId);
}
}
If (solutionCaseMap.size() > 0)
{
Map<Id,string> caseFeatureAreaMap = New Map<id, string>();
for (Case c : [Select Id,Feature_Area__c from case where id in (Select CaseId from casesolution where SolutionId in :solutionCaseMap.keySet())])
{
caseFeatureAreaMap.put(c.id, c.Feature_Area__c);
}
List<CategoryNode> categoryNodeList = New List<CategoryNode>();
for (CategoryNode cn : [Select Id,MasterLabel from categorynode])
{
categoryNodeList.add(cn);
}
List<CategoryData> category = New List<CategoryData>();
for (integer x = 0; x < categoryNodeList.size(); x++)
{
for (integer i = 0; i < solutionIdList.size(); i++)
{
If (caseFeatureAreaMap.get(solutionCaseMap.get(solutionIdList[i])) == categoryNodeList[x].MasterLabel)
{
CategoryData cd = new CategoryData();
cd.CategoryNodeId = categoryNodeList[x].id;
cd.RelatedSobjectId = solutionIdList[i];
category.add(cd);
}
}
}
insert category;
}

}

 P.S. -- What's the trick to pasting code into a post without all the formatting screwing up?? Woot figured it out! Firefox good, Chrome bad :P