Usually, failing fast in software development refers to errors in code, but another interpretation of this is "What's the quickest and simplest way to validate or prove an idea"?
If it's fixing a bug or architecting a new feature, what's the simplest thing that can be done to achieve that? It could be writing a technical document or creating a simple proof of concept rather than building the entire feature.
If it fails, your sunk cost is low as you've not invested much time, money or energy.
