Is is safe to use MtEtM? (MAC-then-Encrypt and Encrypt-than-MAC)
If the MAC is theoretically secure, but its implementation has a side channel (like by Differential Power Analysis, maybe even timing) leaking information about the MACed message (but not the key), and the encryption is secure including it's implementation, then neither MtE nor MtEtM are safe (because the side channel leaks about the message), but EtM is secure (because the MAC is applied to ciphertext, which can safely leak).
Thus EtM can be secure when MtEtM is not. A better reason to prefer EtM to MtEtM is Occam's razor.