The bug
All of the spider's legs begin to bounce up and down for a couple of seconds after pointing my crosshair at it, happens the same way for both kinds of spiders. The bouncing ceases after a couple of seconds or after backing away from the spider, no matter how far away it was when I pointed the crosshair at it. Curiously, it doesn't happen in F1 mode when the crosshair isn't visible. The bouncing also doesn't occur when the spider is moving, only when it's idle or just turns around in place.

Steps to reproduce the behavior:

1. Go to any world
2. Spawn or find a spider
3. Point the crosshair at it
4. See error

Expected behavior
The spider's legs should all bounce up and down until the crosshair is moved away.

Context:

• Minecraft version: 1.12.2
• Mo' Bends version 1.0.0-beta-2
• Lots and lots of other mods, but none that also change animations

Alright, I tried finding out which mod causes the bug to happen. Turns out the bug is caused by Damage Indicators, at least when the "show portrait" option is enabled.

New context:

• Minecraft version: 1.12.2
• Mo' Bends version: 1.0.0-beta-2
• additional mods:
  • Optifine HD U E3
  • Damage Indicators
• Forge version: 14.23.5.2847