Consider the attached two files.  A reader and writer with the same dialect parameters (escapechar \ quotechar " doublequote False) read, then write a CSV cell that looks like "C\\".  It's written "C\".  The problem is, when doublequote=False, the escapechar isn't used to escape itself, and the writer writes something that in the same dialect would be understood differently (\" isn't \ then end of string, it's an escaped quotechar within the string).

Execute python err.py first.csv to see.