‘‘Coreless defects’’ are one of the long‐standing surprises of silicide epitaxy on Si. For symmetry reasons a NiSi2or CoSi2film cannot grow over a step on Si(100), but must incorporate a dislocation; apparently, however, very thin films avoided this dislocation by introducing a trench through the silicide, the coreless defect. Here we use high‐resolution microscopy, electron diffraction and dark‐field imaging to show that these defects are in fact microtwins ≊4 atomic planes thick. The twins are highly unusual in that they follow curved lines, apparently dictated by steps at the interface. A mechanism for the formation of these curved twin defects by repeated formation of a 1/12⟨111⟩ interfacial partial at steps is postulated.