diff --git a/xbstrap/base.py b/xbstrap/base.py
index 508030f..939736f 100644
--- a/xbstrap/base.py
+++ b/xbstrap/base.py
@@ -736,6 +736,12 @@ def __init__(self, cfg, induced_name, yml):
         else:
             self._name = induced_name
 
+        if "git" in self._this_yml:
+            if "branch" in self._this_yml and "tag" in self._this_yml:
+                _util.log_err(
+                    f"source '{self._name}' has both a tag and a branch; this is ambiguous"
+                )
+
         if "regenerate" in self._this_yml:
             for step_yml in self._this_yml["regenerate"]:
                 self._regenerate_steps.append(ScriptStep(step_yml))