Process stability in the thin strip casting process of steel has been investigated with a laboratory pilot caster. Depending on the steel grade and the process parameters irregular process states could be observed. It has been proved experimentally that the observed behaviour is not caused by the regulation system. Investigations of microsections on the cast strips have shown evidence of periodical changes in heat flux during solidification and strip formation as the reason for observed oscillating process variables. Similar to conventional flat rolling forward and backward slip between strip and casting rolls during the joining of the shells can occur. Under certain combinations the process parameters and the cast material a backward‐slip can lead to local lift‐offs of the solidified shells. This circumstance significantly disturbs the heat flux and consequently the solidification, even periodically.