Despite questions about academic rigor, undergraduates use Wikipedia for opportunistic learning: self-regulated study to learn, relearn, or be introduced to concepts. STEM topics are potentially immune from some of Wikipedia's criticism, as many are based on well-established facts. This article evaluates 28 articles from seven STEM disciplines on four criteria important for undergraduate opportunistic learners. Some articles contained inaccuracies, while common criticisms included poor conceptual development, poor procedural explanations, poor diagrams and failing to utilize internet technologies. Often, the best advice is to read the first paragraph: the rest of the article generally did not enhance opportunistic learning.