It is a common error to believe that "correct" in English translates to "korekta" in Esperanto. The correct word is "ĝusta" or "senerara". But if "korekta" does not mean "correct", what does it mean and when should I use it?
The word "korekta" is derived from the verb "korekti", which means "to correct". Therefore its only regular meaning is "characterized by or related to the act of correcting". For example, one can say "korekta laboro" in the sense of "correction work", i.e. the work that consists of correcting something. PIV has another good example: "la korektaj reguloj de presejo", i.e. the rules that a printing house has defined concerning how to correct manuscripts. Note, however, that this regular meaning of "korekta" is only very rarely needed.
Also note that while it is right to say that "correct" cannot be a regular meaning of "korekta", "korekta" has been used in the sense of "correct" by many competent Esperanto speakers, including Zamenhof himself (in his late years). So one can say that "korekta" does in the actual usage have this meaning, even though it is not a regular meaning.
But also note that many competent Esperanto speakers avoid the usage of "korekta" in the sense of "correct" because of its irregularity, as they want to contribute to Esperanto usage becoming more in line with the general rules. The most respected dictionary (PIV) and the most respected grammar (PMEG) both recommend avoiding "korekta" in the sense of "correct". Instead, one should use "ĝusta" or "senerara", depending on context.
According to PIV korekta means:
1 Rilata al korekt(ad)o: la korektaj reguloj de presejo; korekta domo (establejo, kie oni provas korekti k plibonigi junajn deliktulojn).
2 (evi) = laŭregula, ĝusta: via desegno ne estas sufiĉe korektaᶻ; paroli en iu lingvo korekteᶻ.
Thus korektaj reguloj are rules for correcting things: ‘correcting rules’, not ‘correct rules’.
Note that the second meaning does indeed coincide with the meaning of ĝusta. However, PIV marks this as evitinda (worth avoiding).
According to J.C. Wells' Esperanto Dictionary (3rd impression, 1977), the meaning of korekt-i/a is indeed correct in English. He also lists korekta as an Esperanto translation of correct, so I don't know whether it is right to say this is a common error?