With advances in generative AI, there is increasing work towards creating autonomous agents that can manage daily tasks by operating user interfaces (UIs). While prior research has studied the mechanics of how AI agents might navigate UIs and understand UI structure, the effects of agents and their autonomous actions-particularly those that may be risky or irreversible-remain under-explored. In this work, we investigate the real-world impacts and consequences of mobile UI actions taken by AI agents. We began by developing a taxonomy of the impacts of mobile UI actions through a series of workshops with domain experts. Following this, we conducted a data synthesis study to gather realistic mobile UI screen traces and action data that users perceive as impactful. We then used our impact categories to annotate our collected data and data repurposed from existing mobile UI navigation datasets. Our quantitative evaluations of different large language models (LLMs) and variants demonstrate how well different LLMs can understand the impacts of mobile UI actions that might be taken by an agent. We show that our taxonomy enhances the reasoning capabilities of these LLMs for understanding the impacts of mobile UI actions, but our findings also reveal significant gaps in their ability to reliably classify more nuanced or complex categories of impact.