Right-clicking a pig or strider with a saddle at the moment it grows up will saddle and mount it simultaneously without playing a hand animation.

Steps to reproduce:
1.

/summon minecraft:strider ~ ~ ~ {Age:-20}

2. Right-click the strider precisely 1 second later while holding a saddle. This may require a few attempts.

What I expected to happen was:
Right-clicking the strider when it grows up saddles it without mounting it and triggers the hand-swinging animation.

What actually happened was:
With just one click, you saddled the strider and mounted it, and neither action played the hand-swinging animation.