<blockquote>> This could indeed happen, when on a target platform a pthread_t were a complex struct</blockquote>

No. As I already wrote, this can also happen with scalar data (e. g. 64-bit int or pointer on 64-bit platform) if the variable is not properly aligned in memory, e. g. placed in a structure without alignment after some members not aligned to 64-bit (e. g. after int16 or int32). However it depends (platform, CPU, etc).