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Encoding a (well-formed) Unicode string containing a non-BMP character, using the xmlcharrefreplace error handler, will produce two XML entities for surrogate codepoints instead of one entity for the actual character.
Here's a transcript (Python 2.7.3, x86_64):
 >>> b = '\xf0\x9f\x92\x9d'
 >>> u = b.decode('utf8')
 >>> u
 u'\U0001f49d'
 >>> u.encode('ascii', errors='xmlcharrefreplace')
 '��'
 >>> ( u'\U0001f49d' ).encode('ascii', errors='xmlcharrefreplace')
 '��'
 >>> list(u)
 [u'\ud83d', u'\udc9d']
 >>> u.encode('utf8', errors='xmlcharrefreplace')
 '\xf0\x9f\x92\x9d'
The utf8 bytestring is correctly decoded, and the print representation shows one single Unicode character. Encoding using xmlcharrefreplace produces two XML entities, which is wrong[1]: a single non-BMP character should be represented in XML as a single entity reference, in this case presumably '💝'.
As the last two lines show, I'm using a narrow build (so the unicode strings are represented internally in UTF-16, I guess). Converting the string back to utf8 does the right thing, and emits a single UTF8 sequence representing the supplementary-plane codepoint.
(FWIW, the backslashreplace error handler also emits a surrogate pair, but I don't know if there is a complete specification for what that handler does, so it's possible that it's not wrong.)
[1] http://www.w3.org/International/questions/qa-escapes#bytheway