What are the most important security issues that developers of Android apps need to know about? What are the biggest pitfalls or kinds of vulnerabilities that they need to watch out for? One security problem per answer, please. I'm particularly interested in implementation-level issues, software flaws, etc.
Edit: I am most interested in implementation-level failures (rather than conceptual/architectural/design flaws), and flaws that are specific to Android apps (rather than flaws that apply to all application software or all network-connected client software). Are there pitfalls in using the Android APIs that Android developers need to know about? I am also interested in answers that explain how the flaw manifests in the code, or what to look for in the code to recognize the flaw. (Example code snippets would be especially wonderful.)