Nifty halakha

From Mishneh Torah, Hilkhot Issurei Biah, 1:18

א,יח וכן השוכב עם בהמה בשגגה, והאישה שהביאה את הבהמה עליה בשגגה--אין הבהמה נסקלת על ידן, ואף על פי שהן גדולים.

Roughly:

One who lies with a beast accidentally, and a woman who lies with a beast accidentally -- the beast is not stoned[, even if they were adults?]

So, this begs the question: How does one "accidentally" have sex with an animal?