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Consider a multithreaded application that encounters an error while calling a system function. The strerror() function returns a human-readable error string given an error number. The C Standard, section Section 7.24.6.2 [ISO/IEC 9899:2011], specifically states that strerror() is not required to avoid data races. Conventionally, it could rely on a static array that maps error numbers to error strings, and that array might be accessible and modifiable by other threads. (This code is specific to POSIX because fopen() is not guaranteed to set errno if an error occurs in C99 or C11.)
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