Rather than aborting to the nearest level or to the previous level, these cause a reader error of `Illegal character: #\U+00'.  It seems that control never even reaches emacs.scm's hook to clean the input stream flushing typeahead.  I think that control first passes through the reader, which reads the 0 delimiter from the input stream and signals an error, which pushes a REPL level and causes all pending thread events (of which the invocation of the hook to clean the input stream is one) to be discarded, but I have not thoroughly confirmed this hypothesis.