Not a security issue, no.  This isn't C where a stack overflow can give an attacker a vector for injecting arbitrary code.

Per the Parser contract ("raise no exceptions, only register defects"), this should, as you say, register a defect (email.errors.InvalidMultipartContentTransferEncodingDefect) and assume a CTE of 7bit for the rest of the parsing.  The problem here is that the feedparser is running into the "hack" I put in place in python3.2 for dealing with invalid binary data in headers (which is to turn it into a Header with charset unknown-8bit).  That works most of the time, but in cases like this it breaks down :(

Note that the new API (policy=default and friends) handles this without error.