Contributions are always welcome, but surprise patches are mostly just a burden. Yes, you are offering your help, but someone else will have to maintain your code over the long term - get their buy-in first, avoid surprises. Even worse, a localized change to address a specific problem will often miss the full implications to the project: other existing use cases, future roadmap plans, or overall architectural decisions. A good idea can be implemented inappropriately for the specific project; it can be invalidated by another effort that you may not even be aware of; the timing may be wrong, and a dozen other reasons can conspire against you.
This is so true. I’ve had this happen numerous times with RestSharp. Unfortunately more often than not this ends up with the (almost always first-time) contributor becoming disillusioned and giving up on participating in the project. A net loss for everyone.