A scenario I've pointed out to folks many times over the years is humans introduce something that knocks out an important link in the local ecosystem. And before you know it, there's chain reaction leading to a massive die-off.

Which leaves the colonists in a *real* bad spot if it killed off enough of the photosynethsizing organisms.

Besides the massive piles of rotting vegetation (and animals) they'll have oxygen levels dropping (especially if lightning or something starts fires in the dead
vegetation)

If ships don't visit very often you could lose the colony. And have people fighting for places on the ships that do show up.

Yeah, it's a low probability scenario. But it'd only have to happen *once* become a legend and a warning.