In software engineering of self-adaptive systems, uncertainties arise from the inherent complexity of these systems (i.e., the limitation to foresee all aspects related to their development and operation) and the unpredictability of the execution environments (i.e., the limitation to predict all the events), among other sources. Several taxonomies and specification languages have been proposed for uncertainties. However, they remain at a theoretical level and are hardly applied in practice. There is a lack of best practices and concrete examples of how to apply taxonomies and languages in real systems. To address this gap, we contribute a catalog of uncertainties. All the uncertainties in the catalog have been extracted from concrete artifacts. Each uncertainty is described using the same template and associated with requirements specified in the RELAX language. Our objective is to provide an initial version of a catalog (to be further extended) of uncertainties and requirements that can be (re)used as prototypical examples in different application domains.