  Prediction markets provide an efficient means to assess uncertain quantities
from forecasters. Traditional and competitive strictly proper scoring rules
have been shown to incentivize players to provide truthful probabilistic
forecasts. However, we show that when those players can cooperate, these
mechanisms can instead discourage them from reporting what they really believe.
When players with different beliefs are able to cooperate and form a coalition,
these mechanisms admit arbitrage and there is a report that will always pay
coalition members more than their truthful forecasts. If the coalition were
created by an intermediary, such as a web portal, the intermediary would be
guaranteed a profit.
