diff --git a/belay/__init__.py b/belay/__init__.py
index 509eb2e..4b56d2a 100644
--- a/belay/__init__.py
+++ b/belay/__init__.py
@@ -2,6 +2,7 @@
 __version__ = "0.0.0"
 
 __all__ = [
+    "AuthenticationError",
     "minify",
     "Device",
     "SpecialFunctionNameError",
@@ -9,4 +10,5 @@
 ]
 from ._minify import minify
 from .device import Device, SpecialFunctionNameError
+from .exceptions import AuthenticationError
 from .pyboard import PyboardException
diff --git a/belay/device.py b/belay/device.py
index 64ce470..ad6095a 100644
--- a/belay/device.py
+++ b/belay/device.py
@@ -331,6 +331,7 @@ def sync(
                 src_files.append(src_object)
         dst_files = [f"/{src.relative_to(folder)}" for src in src_files]
         dst_dirs = [f"/{src.relative_to(folder)}" for src in src_dirs]
+        dst_dirs.sort()
         keep = [x for x in keep if x not in dst_files]
         if dst_files + keep:
             self(f"for x in {repr(dst_files + keep)}:\n all_files.discard(x)")
diff --git a/belay/exceptions.py b/belay/exceptions.py
new file mode 100644
index 0000000..e75af73
--- /dev/null
+++ b/belay/exceptions.py
@@ -0,0 +1,2 @@
+class AuthenticationError(Exception):
+    """Invalid password or similar."""
diff --git a/belay/snippets/sync_begin.py b/belay/snippets/sync_begin.py
index 93a64ff..44e4bcc 100644
--- a/belay/snippets/sync_begin.py
+++ b/belay/snippets/sync_begin.py
@@ -17,7 +17,7 @@ def __belay_hfs(fns):
 def __belay_mkdirs(fns):
     for fn in fns:
         try:
-            os.mkdir('%s')
+            os.mkdir(fn)
         except OSError as e:
             if e.errno != errno.EEXIST:
                 raise
diff --git a/belay/webrepl.py b/belay/webrepl.py
index 477b63f..64f9a34 100644
--- a/belay/webrepl.py
+++ b/belay/webrepl.py
@@ -24,13 +24,14 @@
 THE SOFTWARE.
 """
 import errno
-import os
 import socket
 import struct
 import sys
 from collections import deque
 from pathlib import Path
 
+from .exceptions import AuthenticationError
+
 # Treat this remote directory as a root for file transfers
 SANDBOX = ""
 # SANDBOX = "/tmp/webrepl/"
@@ -293,8 +294,11 @@ def __init__(self, uri, password, read_timeout=None):
         self.ws = Websocket(self.s)
 
         login(self.ws, password)
-        if self.read(1024) != b"\r\nWebREPL connected\r\n>>> ":
-            raise WebreplError
+        response = self.read(1024)
+        if response == b"\r\nAccess denied\r\n":
+            raise AuthenticationError(f"Incorrect password: {repr(password)}")
+        elif response != b"\r\nWebREPL connected\r\n>>> ":
+            raise WebreplError(f"Unknown login response: {response}")
 
     def close(self):
         if self.s is not None: