FCustomThunkTemplates::Array_Get() fails to properly initialize a default item when the index is invalid:
Item = U{};
That will leave the item uninitialized (e.g. UDS types). Should be using the TArray's Inner property to call InitializeValue() instead.
(See UDN thread)
Basically, set up a BP in which you have an array of user-defined struct elements, and then use a 'Get' node to attempt to access an invalid index. The "default" item returned will not be fully initialized in the nativized build.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-57955 in the post.