comment you want to reply to, by user C:

B said:

A said:
text_a

text_b

text_c
[quote]B said:[quote]A said: text_a[/quote]text_b[/quote]text_c

1. you press relply and the program notices it begins with a quote
2. it removes text up to the first [/quote] to keep comments clean:
text_b[/quote]text_c {your reply}
3. it quotes the text:

C said:
text_b

text_c[/quote] {your reply}
[quote]C said: text_b[/quote]text_c[/quote]
4. now it's fucked. C didn't say text_b.

how it should look:

C said:
text_c

{your reply}