Is error propagation desired in modes of operation I'm taking a very simple class on cryptography. The class is not mathematical is just the very basics. We learned about modes of operations recently (such as CBC and CTR), one of the things I learned was about error propagation. I just don't quiet understand if this is a desired effect or not. Sorry for such a simple question, but unfortunately my class doesn't have textbooks, it's just slides, and when I have questions is difficult to look for answers. Thank you.
It is neither desirable nor undesirable on its own.  Error propagation is not relevant to any modern cryptography.  The concept is a relic of an archaic school of crypto engineering that has been left to the junk heap of the previous century, but not all textbooks or instructors with job requirements to teach courses called Cryptography 101 have kept up with the state of the art.

What is relevant is authentication.  If there is any chance that an adversary might have the power to interfere with the channel between two peers—whether by intercepting packets on the wire or by compromising the server where messages or stored or what—then the peers should have a system for authenticating messages so that any changes an adversary makes have negligible chance of going undetected by the recipient.

If the peers already share a secret key that they could use with a block cipher, then, instead of picking a block cipher and a mode of operation as last century's school of engineering prescribes, they should pick an authenticated encryption scheme.  This is a gizmo that takes a key, a message sequence number, and an arbitrary bit string as a message, and turns it into a ciphertext which the recipient can use the same key to detect modifications in and, if unmodified, turn back into the message.

One example of an authenticated encryption scheme is NaCl/libsodium crypto_secretbox_xsalsa20poly1305.  Another example is AES256-GCM.  AES256-GCM happens to be built out of the block cipher AES-256, but this is an implementation detail—you should think of it as picking AES256-GCM as a unit, not picking AES and then a 256-bit key and then GCM as a mode of operation.  crypto_secretbox_xsalsa20poly1305 is not built out of a block cipher at all, and has some advantages over AES256-GCM—faster and safer in software, larger message sequence numbers so you can safely randomize them.