What it is
Box 1 is taxable wages, boxes 3-5 are social security, medicare, and tips, and box 12 carries retirement deferrals and other codes. A mid-year job change means two W-2s, and the year-end state lines follow where you worked. A severance W-2 line can fall in the year paid, not the year earned.
What to do about it: Collect the W-2 from every employer before your systems access ends. A missing W-2 stops the filing, and a second W-2 with a duplicate SSN is the usual mismatch.
Plain-language description of US practice, not legal, tax, or financial advice. The paired guide carries the citations, dates, and thresholds; the state pages carry the local windows.