I think the most common mistake is to misunderstand just when the salary negotiation should, and does, take place, and the role your current salary plays in the salary offer that will come later.
As a result the candidate often feels defensive and says, “I made $45K but I was there 10 years and there were people hired after me and they made $X, and I felt that I was taken for granted as a long term employee…”
But you’re beginning an oblique argument for your salary, 1) prematurely, 2) from a defensive posture, and 3) before you’ve had an opportunity to make a case why you are the perfect candidate.
Answer the question forthrightly, honestly, and with confidence; and proceed with the interview, understanding that A) the salary offer will come later, (and the negotiations) and B) your current salary is one part of their strategy. (the others being supply and demand for that position, and therefore what the market going rate is)
Your job then, is to wow them so that your current salary is less of a bargaining chip for them. Make them want you.
Your second job is to be really well informed. Know what the market rate is for the job, using credible cites. Make a case why you were underpaid,* (only if it comes up!) *professionally and without sounding like you were disenfranchised by your pay. Certainly don’t make it look like you’re primarily motivated by pay. You don’t want them to see you as a job hopping hired gun.
The question as to your current pay will be early in the interview when you are at your *weakest. * Answer the question and move on. Start the discussion at the appropriate time; at the end of the interview, when you’re in the *strongest *position you’ll be in.
Employers aren’t stupid. They know the appropriate salary ranges for certain levels of performance. If you’ve made your case in the interview, and made your case during salary negotiations, they’re not going to let a good candidate get away, even if means doubling your salary.