The bug

Invisible spiders are special in that their eyes are still visible and glow. However, if they are in front of a sign, the sign will clip through the eyes, making them invisible. Happens to dyed signs, signs of any type, regular spiders, and cave spiders.

Affected Blocks

• Signs
• Beds
• Chests
• Shulker Boxes
• Mob heads/skulls
  List may be incomplete

How to reproduce

1. Place a sign
2. Summon a spider next to the sign, using the following command:

   /summon minecraft:spider ~ ~ ~ {NoAI:1b}

3. Splash the spider with invisibility
   → The spider's eyes render behind the sign