What's wrong with the Pokeball?

I was on Route 13, and I was just fishing. I saw a Shellder, and because I didn’t have one, I decided to catch it. The thing is, when I threw my Ultra Ball on the first turn, it shook once inin the airbefore landing, and as soon as it landed, it darkened and was counted as being ‘caught’. Anyone experience the same thing or have a thought?

Pleaselog inorregisterto add a comment.

It’s called a critical capture. It happens and it’s lucky. It’s rare and is almost a guaranteed capture. You’re lucky to get it. Source experience.

Critical Capture

A totally new mechanic of capture is the Critical Capture. This feature works like Critical Hits in battle where there is a slight chance that this feature occurs. Critical Capture will be noticed when you throw the Pokéball. It’ll pause in mid-air and make a metal noise. When it hits the Pokémon, it will shake once and capture. This cuts two of the four random calculations out of the overal capture mechanic making it more likely to capture the Pokémon. However, it can still fail.

The calculation is done by a simple factor depending upon the amount of Pokémon species you have captured. It will run this calculation based upon the capture value calculated above and will then have another random number based upon the Pokémon you have in your pc.

http://bulbapedia.bulbagarden.net/wiki/Catch_rateThis explainers the chance of it happening.

It was a critical capture. They happen rarely in B/W. They almost always catch the Poke’mon.Source: I read it on this site, and it happened to me. A weird noise happens too, if you have the volume up.