Inter-canister calls do not have a delivery guarantee, maybe C2 was removed or upgraded to remove the specific method before the invocation message arrived.
But inter-canister calls do have a response guarantee, so eventually a response to the inter-canister call will make it to C2. The response may be a reply/reject from the invoked canister, or it may be a system-generated rejection. And there is no fire-and-forget, all inter-canister calls expect an answer.
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