For example, in a machine translation system (Koehn et al., 2007), if the bilingual training data does not contain the word " " (the second example in Table 1), it leaves the word untranslated.
if the word " " does appear in the training data but it has only a translation "gruel" as that is the meaning in the formal text, the translation system may wrongly translate " " into "gruel" for the informal text where the word " " is more likely to mean "like".