In order to save this blog from being just "Jonathan posts funny signs from offices" here is one of my irregular posts about a common houseplant pest. This one is a little swine, thankfully not the most common, but very tenacious when you do have it.
Hard scale is a part of the larger scale family that also includes my favourite Snow Scale and also Mealy Bug, which many people don't realise is a type of scale at all.
Now, the reason hard scale is a particularly tenacious pest is because it is hard! As you can see in this image they somewhat resemble barnacles.
This means that scale is well-armoured (so nonabsorbent) against any pesticide applications etc. In practice the younger softer scales may be killed, but not the harder ones, so the key if using pesticides is repeated applications.
I am told that rubbing alcohol is also quite effective. But I have found the best method is physical removal, to whit scraping them off with a finger.