Back in the 1980s British Airways (and others- PAA and TWA I think) had three incidents of flap tracks breaking on their older 747-100s. The cause was corrosion pits in the bolt holes acting as stress raisers, resulting in a flap track breaking. I cannot remember whether this occurred only on the inner sets of trailing edge flaps or on the outers. The breakage occurred when flap 30 was selected and in each case the crews elected to continue the approach and landing. In each case the flap set remained attached and the aircraft could be held wings level with the control spectacle rotated through about 90 degs (I cannot remember whether some rudder was required too). In each case the aircraft landed safely.