.net – safely install .Net framework 3.5 SP1 without requiring the customers to upgrade their .Net Framework distributable? (currently running 3.5)

I just had the same problem, and I found a solution. For our application, it was a call to System.Threading.WaitHandle.WaitOne(Int32) that got us in trouble. For more details on how references to API's that were introduced in service pack releases can leak into your code without Visual Studio noticing, see Krzysztof Cwalina's post.

The good news is that, as Marc mentioned is his answer, FxCop has a new rule that detects these leaks. The bad news is that the rule is broken in FxCop 1.36 when you target .NET Framework 3.5. However, David Kean describes how to edit a couple of XML configuration files to fix the problem. I followed the instructions, and FxCop now detects my references to service pack API's.

They don't know where you're starting from, so it's a full cumulative update. That's why it's so big. It's all the patches to go 2->3->3.5->sp1. It'll just use what it needs to.