A developer encountered a baffling error in the Karafka gem, where a `NoMethodError` for a String indicated that an internal Hash had somehow transformed into a String. The issue arose from FFI versions below 1.17.0, which lack proper write barriers, causing Ruby's garbage collector (GC) to free internal Hashes. As a result, the memory previously allocated for a Hash could be reused for a String, leading to the bizarre behavior of the program. The developer faced 2,700 error reports in a single incident, highlighting the severity of the problem. The investigation began with examining the error's context, particularly at line 112 in the rdkafka-ruby code. The FFI::Struct was correctly defined, and the developer ruled out basic issues like type mismatches and struct alignment. They focused on the internals of Ruby and FFI, discovering that accessing a field in the FFI::Struct relies on a Hash that stores field definitions. If this Hash were freed by the GC, the memory could be repurposed for a String, resulting in the unexpected call to `#default`, a method exclusive to Hashes. The developer suspected that the use of musl libc in the Docker environment might be a factor, but tests confirmed that struct sizes and alignments were as expected. Ultimately, they connected the dots to the missing write barriers in FFI's C extension. The absence of these barriers led to the GC not recognizing all references, allowing the Hash to be improperly freed. A reproduction script confirmed the issue, showcasing how transient struct classes could lead to similar segfaults, emphasizing the critical need for better memory management practices in the FFI library.