The MYSQL structure in the server is not the same as in C/C — they diverged when MEM_ROOT got PSI_memory_key member and thus became larger than MA_MEM_ROOT. This means that all offsets in MYSQL after the memroot are different in the server. And if a server plugin (which uses server's MYSQL) needs to load a client plugin (which is part of C/C and uses its MYSQL definition), the plugin might corrupt caller's MYSQL.