I am using Freeling to process English sentences, more particularly using hmm_tagger.
At the beginning I add a lot of errors "TAGSET: No rule to get short version of tag [tag]" where tag was of the form "tag1+tag2" (e.g. : 'DT+MD', 'VB+PRP'... It mainly corresponds to all the English abbreviation I'll, We'd, you're...).
Even if they don't make the program crash, those errors was just overflowing the terminal, slowing the program, and it seems that those PoS tags was not assigned to corresponding words.