Someone I know was powdering her face and decided to do me the favor of applying some to my face as well. She then laughed and said I will get pimples because I don't have something to remove the makeup with later. She thought that the powder will block my pores, they'll get infected and I'll get pimples. What basis is there for that claim?
Older formulations of makeup may cause acne by blocking pores; this is known as acne cosmetica (see Wikipedia, Skin Channel, About.com Acne, WebMD). Many modern cosmetics are marketed as "non-comedogenic", which means that they do not block pores and therefore are not likely to cause pimples, although there are still some products that contain comedogenic ingredients and can cause acne.