Harmonization is not wrong in itself, and it is appropriate to harmonize the myriad of witnesses in the bible in order to understand their collective human witness to the one and only Jesus Christ. However, there is a ridiculous form of biblical harmonization that is practiced by people who are convinced that the bible is absolutely free from every possible contradiction or error. (As if one tiny geographical, mathematical, zoological, historical, or theological error would disprove that Jesus Christ was crucified and resurrected). 

James Barr in his book Fundamentalism defines this ludicrous biblical harmonization as follows:

If two passages in the gospels describe in different terms what seems to be the same incident, they are harmonized in the conservative literature. The most common way to do this is to add the two together, so that what one says complements what the other says. Certainly it is admitted that one evangelist has seen things or described things rather differently from another, just as two persons who witness the same road accident will describe it differently. But they cannot be in real contradiction, they cannot be saying really different things that cannot be reconciled. [1]

A long time ago when I was young, I was frustrated by a theologian who would not admit that the slave's name in Mark 14:47 was "Malchus" because John 18:10 says Peter cut off the Malchus' ear. I could not fathom why this theologian would not harmonize these two accounts, because the possibility that John's scribal note was an error was unthinkable. What I didn't understand is that the differences, including the errors and contradictions, strengthen the independent witness of Mark and John, and collusion of the two create a false history that never happened. 

In the worst cases of harmonization, false history is invented in order to harmonize two contradictory events, and the result is worse than the two biblical difficulties the harmonization wished to explain away. James Barr recalls Albert Schweitzer's famous and infamous discovery of Markan priority when Schweitzer demonstrated that Jesus cleansed the temple at the end of his life (as in the Gospel of Mark), and not at the beginning of his ministry (as in the Gospel of John). Barr explains that biblical harmonization (as practiced by proponents of biblical inerrancy) conclude that Jesus cleansed the temple twice, rather than admit that the cleansing did not happen at the beginning of Jesus' ministry as reported by the Gospel of John. 

The most striking example is the famous cleansing of the Temple by Jesus. In the synoptic gospels this is narrated at the very end of the ministry of Jesus, at the beginning of passion week (Matt 21:10-17; Mark 11:15-19; Luke 19:45-48), while John has it right at the beginning of the ministry (John 2:13-17). The New Bible Commentary Revised (on Mark, C. E. Graham Swift, p. 875b) gives us the simple but ludicrous harmonization: 'By far the most satisfactory solution is that Jesus cleansed the Temple twice.' Why not? By the same account, why should the ascension of Jesus to heaven not have taken place twice? This would successfully harmonize the facts that, according to Luke 24:51, the ascension appears to have taken place on the same day as the resurrection, while Acts 1 expressively makes it about forty days later. Jesus was carried up to heaven, but later returned, appeared and spoke with his disciples for forty days, and then finally ascended again. Why not? I have actually heard this explanation offered in all seriousness by a prominent conservative scholar. 

Harmonization through the production of multiple events is the most thoroughly laughable of all devices of interpretation. [2]

James Barr cites Andreas Osiander's Harmonia Evangelica (1545) as an example of a reformer who argued that Peter warmed himself four times and there were a total of eight denials to harmonize the gospels. This form of 'biblical harmonization' demonstrates when a hermeneutic takes control of the bible and changes its message to fit its own ends, and ceases to help explain the biblical text as we've received it. 

James Barr explains that contradictions and errors are not an embarrassment to the bible that Christians need to cover up and explain away. Barr gives the example of the Documentary Hypothesis, which was discovered by noticing the differences between different accounts in the bible of the same events.

Sources:

1. James Barr, Fundamentalism, Wipf and Stock Publishers, (Eugene: 1977), p. 56.

2. James Barr, pp. 56-57.