Styles scrambled in document

Check if this helps.

http://answers.microsoft.com/en-us/office/forum/office_2010-word/revert-altered-quick-style-back-to-the-style/b5f0ee2f-ea7f-457f-b8cf-b7fac7f67648

The most likely culprit when text acquires an unwanted style is the "Define styles based on your formatting" setting at File | Options | Proofing | AutoCorrect Options | AutoFormat As You Type. If you have that option enabled, disable it at once! If it turns out that that was the cause, then at least you won't experience the problem in future.

As for repairing the document, if Style C has been consistently applied to either Style A or Style B, and if Style C is not used intentionally anywhere in the document, then you can reapply the correct style either using Replace (searching for Style C and replacing with Style A or B) or using Find In (or Select All x Instances if that option is available in the Style pane) to select all the instances of Style C and apply Style A or B as appropriate.

If Style C has been applied to some paragraphs of both Style A and Style B, or if Style C is legitimately used elsewhere in the document, then you'll need to use Find to search for Style C, stepping through the document one instance at a time and reapplying Style A or Style B (from the Style pane or Quick Styles gallery) as appropriate.

Thanks JairOV and Suzanne. That option was not enabled, so I guess we wont easily know why this corruption happened.

Unfortunately, style C is used in the document just as much as styles A and B. It looks like there's no other quick fix except the grueling task of making style-based find&replaces one occurrence at a time