Solve √(x + 3) = x − 3. If you square both sides and solve, you get x = 1 or x = 6. But check x = 1: √4 = 1 − 3 → 2 = −2. That's false! x = 1 is an extraneous solution — a fake answer created by the squaring process.
Squaring both sides can introduce solutions that don't satisfy the original equation. Always check.