In the app releasing process, Android requires all apps to be digitally signed with a certificate before distribution. Android uses this certificate to identify the author and ensure the integrity of an app. However, a number of signature issues have been reported recently, threatening the security and privacy of Android apps. In this paper, we present the first large-scale systematic measurement study on issues related to Android app signatures. We first create a taxonomy covering four types of app signing issues (21 anti-patterns in total), including vulnerabilities, potential attacks, release bugs and compatibility issues. Then we developed an automated tool to characterize signature-related issues in over 5 million app items (3 million distinct apks) crawled from Google Play and 24 alternative Android app markets. Our empirical findings suggested that although Google has introduced apk-level signing schemes (V2 and V3) to overcome some of the known security issues, more than 93% of the apps still use only the JAR signing schemes (V1), which poses great security threats. Besides, we also revealed that 7% to 45% of the apps in the 25 studied markets have been found containing at least one signing issue, while a large number of apps have been exposed to security vulnerabilities, attacks and compatibility issues. Among them a considerable number of apps we identified are popular apps with millions of downloads. Finally, our evolution analysis suggested that most of the issues were not mitigated after considerable amounts of time across markets. The results demonstrate the emergency for detecting and repairing app signing issues.