Glitches are small errors in otherwise goodfic.
Examples of these are a canon character holding his gun the wrong way round (while he should know better), a soldier saluting when he shouldn't or a character appearing, for one line, in a scene where he has no business being. These are the sort of errors that may have just escaped the attention of the beta reader.
Agents Allison and Tasmin have been on several missions to sort out such glitches.