While Fools should generally take the opinion of Wall Street with a grain of salt, it's not a bad idea to take a closer look at particularly stock-shaking upgrades and downgrades -- just in case their reasoning behind the call makes sense.

What: Shares of Carnival Corporation (CCL -4.62%) slipped 1% this morning after Jefferies downgraded the cruise operator from hold to underperform.

So what: Along with the downgrade, analyst Ian Rennardson planted a price target of $33 on the stock, representing about 18% worth of downside to yesterday's close. While momentum traders might be attracted to the stock's strength over the past three months, Rennardson thinks that Wall Street is underestimating the execution risks surrounding Carnival's potential earnings turnaround. 

Now what: According to Jefferies, Carnival's risk/reward trade-off is rather unappealing at this point. "We acknowledge the significant recovery potential in earnings for CCL over the next few years, albeit with significant execution risk, but believe this to be more than priced into the share prices," Rennardson noted. "Our forecasts are based on a 1% fall in yields in 2014, followed by c3% increases in 2015 and 2016." With the stock up more than 30% from its 52-week lows and trading at a P/E of around 30, waiting for a wider margin of safety certainly seems prudent.