A framing error occurs when the Ethernet I/O driver receives a non-integral unit of octets, such as 63 bytes and then 3 bits. (Ethernet specifies the use of octets.) Framing errors are caused by corruption of the starting or ending frame delimiters. These delimiters can be corrupted by some violation of the encoding scheme.
Framing errors are a subset of CRC errors, which are usually caused by anomalies on the physical media. An alignment/framing error is a type of CRC error where octet boundaries do not align.