Most processing errors result from these common mistakes:
Omitting an end tag
Using an incorrect entity name
Referring to an invalid element ID
Omitting an end tag for an element is a common mistake. When creating elements, such as a list, figure, note, caution, or warning, be sure to include the end tag. Check your markup carefully especially if you have nested one element within another, such as a figure within a list,
Errors can also be introduced by using an incorrect entity name. In most instances, it is simply a misspelled word. In other cases, an entity name may have been changed, but cross-references to the original name were overlooked. When you change an entity name, remember to search your source file (or files) for all instances of the entity name.
Similarly, changing the ID assigned to an element affects any cross-reference or link to that topic.