Britney Spears ended her world tour on a sour note Sunday night when security concerns forced her to cut short her final show in Mexico City. In the middle of singing her fifth song — oddly enough, "Stronger" — Spears apologized, saying, "I'm sorry Mexico. I love you. Bye," Mexico City's Milenio newspaper reports. Minutes later, fans allegedly began hurling Spears-related merchandise and chanting "fraud, fraud." Concert organizers — who will offer patrons a full refund beginning Thursday — said that an approaching thunderstorm was to blame. All this comes just days after the 20-year-old was caught flipping the bird to a crowd gathered at an airport in Toluca, about 40 miles from Mexico City. She later insisted that the gesture was meant for the intrusive paparazzi — not her Mexican fans.