There is a long tradition of removing contentious passages in famous (I hesitate to say great) works. The most famous example that I know of is Thomas Bowdler's edition of Shakespeare, wherein he (or his sister Harriet) removed objectionable passages in Shakespeare and then published the sanitized result as The Family Shakespeare. The act was so infamous that his name survives in the term "bowdlerization."
Another act of bowdlerization can be found in the school editions of Rizals' novels. His novels, Noli Me Tangere and El Filibusterismo are required reading (there's actually a law for it) for all secondary and university students. His novels were written as a protest against the abuses of the Spanish colonial government and the friars; these novels were the impetus for the Philippine revolution. (Ironically, Rizal was a reformist, and he believed in making the Philippines a province of Spain, where all indios were to be recognized as Spanish citizens on an equal footing with the peninsulares, that is, those born in Spain.)
When I was a secondary student, I read both novels, and I could not understand why the friars found his novels objectionable. The translation I read was execrable-- we used Maria Odulio de Guzman's translation, and it was written in an extremely stilted form of our national language, Filipino. Everyone had to suffer through it, and some of us escaped reading it by reading the abridged comics version instead.
I have the habit of comparing various editions (and translations) of books that I read, and one of the things that I did as a university student was to buy an English translation of Noli Me Tangere (by Lacson-Locsin). After disliking the novel so much while in high school, I was surprised to find myself (1) enjoying the English translation and (2) encountering passages that I could not remember reading from Maria Odulio de Guzman's translation-- passages, when read, imply the influence of Protestant thinkers. For example, the rivalry between Kapitan Tiyago and an old lady who kept surpassing him in her shows of devotion to the Virgin of Antipolo is lampooned. Another example is a passage where the images of the various saints in Kapitan Tiyago's house is made fun of.
Other passages that would have appalled the friars were those in which Pilosopong Tasyo explained why the idea of purgatory was doubtful, and how the practice of papal indulgences was supposed to be a moneymaking machine for the catholic church. This long sequence of passages, by the way, is missing from Odulio de Guzman's translation; the only thing left of Pilosopong Tasyo's discourse on purgatory was the ironic (or sarcastic?) statement at the end that the idea of purgatory is a good and holy thing.
The preface shows where the bias is. The target audience of the translation is the students of Catholic schools. And so creative subtraction is evident in a work that is barely a hundred years old. Unfortunately, Maria Odulio de Guzman's translation remains of Rizal's novels is still used uncritically in most private and public schools. There is a need for better translations for high school use, but I doubt that there will be changes in the near future, especially in Catholic schools.
Bowdlerization, as well as creative addition and subtraction, is not a new thing. The only good thing about the more recent attempts is the existence of records. We can, if we are patient enough to compare translations, see where such addition and subtraction might take place. Furthermore, we have the recourse of seeing the original editions if we happen to be patient enough, and have the resources to visit the libraries that house the first few editions of the works that interest us.
Not so in scripture. The old manuscripts that we do have are copies of originals (called autographs) that are lost. We have to rely on the biblical scholars (with the requisite knowledge of the ancient languages) who actually do examine the manuscripts, and write critical editions. Most bibles (the pocket sized one that most people own) have no trace of the labor of the translators, nor do they contain notes on the discrepancies between the different manuscripts and translations. It is thus a good idea to have as many translations at hand and to diligently compare them to see where bias might come in. The better translations will contain an apparatus: these are footnotes where the discrepancies between existing manuscripts are noted, as well as alternative readings.
For example, the apparatus of the Jerusalem Bible, a catholic translation, contains a note on Isaiah 7: 14-- the verse is rendered "The Lord himself, therefore, will give you a sign. It is this: the maiden is with child and will soon give birth to a son whom she will call Immanuel." the apparatus (the footnote) says: The Greek version (that is, the Septuagint) reads 'the virgin' being more explicit than the Hebrew (that is, the Masoretic text), which uses almah meaning either a young girl or a young, recently married woman.
Another example of the usefulness of the apparatus can be found in Mark, specifically the apparatus for Mark 16:9-20. A longish note says that the older manuscripts-- the codex Vaticanus and codex Sinaiticus -- do not contain these verses; instead a shorter ending is found. These verses are about the appearances of the risen Christ. The apparatus makes interesting reading, and rather than retyping it here, I recommend visiting a nearby library. My copy of the New Oxford Annotated Bible also has an apparatus, and it also notes this discrepancy between the extant manuscripts.
All this merely shows that one should never put too much trust in books; independent confirmation is always a good thing. As Ronald Reagan would say:"Trust, but verify."