A legal technicality is any aspect(s) of the law or legal process that may affect the outcome of criminal or civil cases. As such, these technicalities vary greatly depending on the type and circumstances of each case. In general, however, there are some that affect the way in which a court rules and others that affect the way in which a court assesses whether or not someone broke the law.
The term legal technicality is often used by someone who is displeased with the outcome of a criminal or civil case.
For example, the prosecutor in a criminal case that has been dismissed due to insufficient evidence may tell the media that the defendant got off on a legal technicality. On the other hand, a defense lawyer who feels that the judge made errors during the trial may also tell the media that they plan to appeal based on legal technicalities.