The article aims to present findings of a study which has examined the ability of elementary school mathematics teachers and of teacher-trainees in other disciplines to solve irregular challenging problems of sequences in general rather than numerical sequences only. The findings show that mathematics teachers succeed to cope with unusual assignments when the requirements of the problems presented to them are analogous to irregular problems. However, when the problems require a change in the thinking procedure in the direction of creative thinking, there is a considerable decrease in performance. Another finding shows that, although teacher-trainees succeed less in solving the presented problems, they give incorrect solutions which do indicate creative thinking. An inevitable conclusion based on the research findings is that teacher training institutions should enhance and reinforce multi-directional. branching out and creative thinking competences.