There are two corner conditions to worry about, not one:
A: You play a goal that you fulfil, causing the potato to rotate to you.
(Do you win before the potato lands?)
B: There is a goal in play which you fulfil, but you also have the
potato. You play a new goal, causing the potato to rotate *away* from
(Do you win right after the potato leaves, before the goal changes?)
And there are three possible answers:
1: The potato moves right after the goal changes. (A: yes. B: no.)
2: The potato moves right *before* the goal changes. (A: no. B: yes.)
3: It's simultaneous. (A: no. B: no.)
The one game I've played with potato in the deck, we ruled
"simultaneous". Mostly because it's easier -- you don't have to reason
about momentary states.
Actually, there is another corner condition:
C: A goal is played which two adjacent players both fulfill, and the
potato is passed from one to the other.
Both players have five Keepers.
Both have the same number of cards, and more than 10.
One player has the Brain and the other has Andy Looney. (Brain: No TV goal)
There are also some possibilities with Double Agenda.
I think I like the 'simultaneous' argument, in which case the player who
ends up without the potato wins.