From what I understand, sometimes you accidentally reformat a paragraph that's in a style, and all instances of that style in the document change to match it.
If so, the Modify Style dialog box probably has the "Automatically update" check box filled. This is only available for Paragraph Styles. (If you reformat only some of the text in the paragraph, it doesn't change the style to match. If you reformat the whole paragraph, it does.) To fix this, select an example of the offending style in the document, then in the Styles and Formatting task pane, select Modify... from the drop-down menu, then reformat it to what you want and clear the "Automatically update" check box.
If the "Automatically update" check box wasn't filled, and changing a paragraph doesn't change all the instances of its style, then I'd use Find and Replace. In the Find and Replace dialog box, click the More button, then Format, Style..., and select the style. (Make sure the cursor is in the box you want; too often I've told it to find something when I wanted to replace it.)
If this doesn't work either, put the cursor in some of the text (either proper or offending), open the Reveal Formatting task pane, check the "Compare to another selection" box, select "Select All Text With Similar Formatting" from the drop-down menu, and if the differences between them are minor, it should find and select them.
Hope something here helps.
(Word: enough features to do anything, including shoot yourself in the foot.)
