The most common is if you create a reference to a field of a struct that is itself weak. The GC will gc the struct and your var can go from non-nil to nil under

The most common is if you create a reference to a field of a struct that is itself weak. The GC will gc the struct and your var can go from non-nil to nil under the hood despite having zero concurrent code (that you wrote). The good news is the vast majority of code using Weak semantics is gratuitous - "clever" code that is actually broken premature optimization, and the fix is to just Don't Do That.