In JET, the ablation of injected pellets produces a striking resonance effect when the pellets reach surfaces with q-values 1 and 3/2. Subsequently, structures with mode numbers m = 1, n = 1 and m = 3, n = 2 are observed with the soft X-ray cameras for more than 2s as compact snake-like perturbations. These structures, which persist through several sawtooth collapses, give information on the radii of the q = 1 and q = 3/2 surfaces and the q-profile evolution. The observations can be explained by the formation of magnetic islands.