Kling-On Acoustic Guitar Removable Teardrop Pickguard, clear
[SS-1P-C]

If your guitar has a nitrocellulose lacquer finish, you have to be super-careful to only put this on while you're actually playing. The vinyl material can interact with the finish, causing permanent damage. The instructions supplied with the product warn you not to leave it attached, but it's easy to forget.
Vinyl does not seem to interact with UV-cured finishes, such as on Taylor guitars. I don't know whether it affects French polish, but better safe than sorry.

*SBM Note: Based on the number of sales, there are very few poor interactions with finishes. Finishes can vary from luthier to luthier and can be unavoidable when the "secret formula" changes. As stated above and a good one, always be careful when applying, and in addition, it is recommended to remove when not in use.