An ASAN use-after-free error was thrown for a stack variable during a run of the "Client reset during schema migration" test, where an UPLOAD message was being processed while the test was being torn down.
https://spruce.mongodb.com/task/realm_core_stable_macos_asan_baas_integration_tests_patch_e019c79e4eae148e58fe45d63f6020fd6ba346d2_6697c13f89647b000709de06_24_07_17_13_04_01/logs?execution=0&sortBy=STATUS&sortDir=ASC

It appears the ASAN failure was likely due to one of the captures is being used in the beginning of the SyncClientEventHook function, but this hasn't been verified:
https://github.com/realm/realm-core/blob/master/test/object-store/sync/flx_schema_migration.cpp#L772-L773

As far as I know, this has only been seen once.