There has been some confusion about catching the disc while the foot is underneath the line. Here is the response from Lorne Beckman, the person who has been involved in all the BULA rules as well as most UPA rules:

First let’s clarify nomenclature: a person's foot cannot be "underneath the line". "Lines" are conceptual. A person's foot can be underneath the coloured tape, but the coloured tape is not the line. The coloured tape is only an aid to mark the lines. See the BULA addendum to the WFDF rules:

• 2.2 The perimeter of the Field of Play is the Perimeter Line and consists of two (2) Sidelines along the length and two (2) End lines along the width.
• 2.2.1. All lines shall be marked with colored tape between five (5) and ten (10) centimetres wide

The conceptual "line" exists as a plane that extends infinitely upward and downward parallel to the pull of the Earth's gravity.A person who catches the disc with a foot which is underneath the coloured tape is therefore squarely on the line, and therefore, out-of-bounds. If the disc is caught by someone with a foot before the tape and then the person’s foot slides underneath it, he/she is in-bounds.