No matter the market, there will always be losers -- a few lagging disappointments holding back a Wall Street rally, or several big losers leading a bearish day. The S&P 500 (SNPINDEX:^GSPC) ended the day by losing a fraction of a percent, but several notable big names helped to weigh down the index and make investors pull their hair out in frustration. Here are the three worst stocks today that you need to know about – from big-box retail to medical robotics, these stocks put a dent in Wall Street's Thursday.

Down go today's losers
Medical robotics maker Intuitive Surgical (NASDAQ:ISRG), a stock that had risen more than 15% year to date, and one of the biggest health-care successes of recent years, put an abrupt end to the party today. Shares of the medical device company plunged 11.1%, just minutes before the markets closed, as a report emerged from Bloomberg that federal regulators  are probing the company's da Vinci robotic surgical system's safety profile.

The FDA is looking to determine whether questionable safety reports sent to the hospital are indicative of a larger-scale problem with Intuitive's device, or whether they're simply due to other unreported factors. Intuitive chief medical officer Myriam Curet defended her company's system as "extremely safe," and the panicky sell-off of this scale might not be warranted, considering regulators are merely asking surgeons at several key hospitals  about any complications from the device. It's worth keeping an eye on this development but, so far, Intuitive has been the shining star of the nascent medical robotics industry.

First Solar (NASDAQ:FSLR) didn't have the kind of explosive and dramatic sell-off that Intuitive did, but this stock's troubles today were enough to rank it among today's three big losers – for the third straight day. Shares of the solar energy company fell another 4.4% after yesterday's gut-wrenching earnings fallout that nuked the stock to the tune of losses of nearly 14%. Over the past five days, First Solar has now lost almost 25%, as the company's guidance -- the big optimism-killer here -- sparked a new round of investor fears. A pair of downgrades has hurt the stock even more and, while the dip might be a temptation for investors looking for a good entry point, First Solar's slipping backlog and modest expectations aren't impressing Wall Street.

Even Intuitive's losses can't compare to those at today's biggest Wall Street victim, however. Shares of J.C. Penney (NYSE:JCP) were decimated by nearly 17% -- the real biggest loser on the S&P 500 -- after the retailer announced a bloodbath of an earnings report. J.C. Penney reported a quarterly loss of $2.51 per share, as revenue fell nearly 25%, with revenue at stores open at least a year -- a key indicator of J.C. Penney's lasting health -- falling more than 31%. Full year revenue for the company also fell around 25%, with earnings-per-share losses of $4.49. Ouch.

The mercilessly painful numbers make it clear that CEO Ron Johnson's turnaround isn't working – at least, not yet. According to a report from Reuters, analysts predict Johnson may have until the back-to-school season this year to prove he can turn J.C. Penney in the right direction; if not, the retailer may be headed for a change of leadership. Either way, the bad news from this stock gives one more reason for investors to stay away.