[REQUEST] What are the odds of captcha generating all the same letters/numbers?

I got this captcha of all J's when tried logging in, just wondering what are the odds of this happening? This particular captcha usually returns either 4 or 5 different numbers and letters, but this is the first time I get all the same letters.

I got this captcha of all J's when tried logging in, just wondering what are the odds of this happening? This particular captcha usually returns either 4 or 5 different numbers and letters, but this is the first time I get all the same letters.