It gets me, over-optimization. Something is either optimal, or not.
Anyway, any keyword stuffing needs to be avoided.
Using bold, underline, and italic for your keywords is overdoing it. (these are markup tools, not content tools now)
Using alt tag descriptions for your images that are keyword related, but not related to the actual image is bad. (instead of saying the image is "best blender under $100" say "man holding blender")
Using heading tags to control font size instead of style statements can inadvertently keyword stuff your heading tags.
If you are writing using keywords instead of popular vernacular is bad:
"Thompson water seal is the best water seal in Texas" is fine.
"Thompson water seal Texas is the best" is not.
(unless your company name is "Thompson water seal Texas")