Unfortunately, I have seen this happen more than once. However, all is not lost, and the situation does not mean your career is over.
Spin Your Situation
It will not necessarily make you look bad to move quickly one time; in fact, your resume will probably speak for itself. Many potential employers have experienced a poor fit themselves. If you have been at the firm only a short while, and you are already looking to move on, a potential employer will probably be able to put two and two together. It will certainly help if you were at your previous firm for a substantial amount of time.
The important thing to think about is how to express your desire to move on without bad-mouthing your current employer or particular partners. You always want to remain professional. Try to spin the move in the most neutral terms possible. For example, did the firm lose a large client that you were hoping to work with? Did a partner who was a potential mentor leave for another firm? Did the firm merge? Are they having publicly-known financial issues? Even if none of these scenarios has taken place, you could state simply, "the work I was hoping for did not materialize, and I have not been happy with the work I am being assigned." Another neutral spin might be, "The work is more specialized than I had understood it would be, and I do not want to be so specialized so early in my career." Don't be afraid to admit that the work is a bad fit. However, I strongly advise that you avoid mentioning any particular partner who is making your life miserable.
It is my belief that associates get no more than one pass in this regard. You are not going to be able to quickly leave a firm the second time this happens; you will be regarded as the problem. This is unfortunate, but it is the reality. Therefore, if this is not the first time, you might consider sticking it out.
Do Your Due Diligence Before Accepting Any Offer
The possibility of ending up in this scenario speaks to the importance of doing one's due diligence before accepting a job offer. It is easy to get caught up in the excitement of the moment, accept what looks like a great offer, and simply hope for the best. Information can be hard to come by, especially, for example, when an attorney is relocating and has few contacts in the new market. Sometimes, attorneys do not want to ask around for fear that their current firm will hear that they are thinking of leaving. However, it is imperative to take all possible steps to ensure that the fit is the right one.
Before accepting any offer, use all available research tools to find information on particular firms, practice groups, or individual partners, whether you are going to be relocating or not. Once you have an offer or feel that you are very close to getting one, get all the information you can. Legal tabloids love to expose partners for saying abrasive things, or law firms who are fighting lawsuits by employees. You can learn about layoffs, client departures, and partner exits. You can learn about a wild holiday party. While this information is not necessarily decisive, you may be able to spot patterns or piece together a firm culture based simply on what you can gather online.
If you are working with a legal recruiter, that person will often have inside information on law firms and be familiar with particular partners and practice groups. You can always ask the group's associates for an informational lunch or coffee; I'm frequently amazed at the things present associates will tell a potential associate. However, it is my opinion that your best resource is attorneys who have firsthand knowledge of the firm. Contact your law school's career services office and see if they can reach out to alumni who have worked in that office (they probably will not supply you with attorneys' contact information, but they may distribute yours so that attorneys can reach out to you). Friends of friends or family may also lead you to someone who will talk to you, or you might have law school classmates who spent time at the firm. Attorneys who worked at the firm but no longer do are probably the best source for honest information. Former paralegals and legal assistants are also wonderful sources.
Once you have lined up people to speak with, you will need to use all your people skills to get them to speak freely. Once you get an attorney on the phone, explain your situation honestly; you have an offer and are trying to be as careful as possible to make sure it is the right move for you. Be humble and appreciative of their time. If they are not comfortable discussing things on the phone, ask to take them to coffee or lunch. Assure them that the information is for your ears only. Also, be mindful of what people are not telling you; if you are told repeatedly that they are not comfortable discussing certain partners, practice groups, or situations; if they raise eyebrows or give a sly smile, this may be a sign of a lurking bad situation. If something doesn't feel right, consider going with your gut. Have your antennae up.
In sum, the best way to avoid being unhappy in your new job is to find out everything you can before accepting any offer. If you do find yourself in an unhappy situation, however, it will not necessarily hurt you if you can spin it in a neutral light, and if you do not make a habit of it.