diff --git a/http2irc.py b/http2irc.py
index 039c078..a3e7ef0 100644
--- a/http2irc.py
+++ b/http2irc.py
@@ -381,12 +381,20 @@ class IRCClientProtocol(asyncio.Protocol):
 		if message.startswith(b':'):
 			# Prefixed message, extract command + parameters (the prefix cannot contain a space)
 			message = message.split(b' ', 1)[1]
+
+		# PING/PONG
 		if message.startswith(b'PING '):
 			self.send(b'PONG ' + message[5:])
 		elif message.startswith(b'PONG '):
 			self.pongReceivedEvent.set()
-		elif message.startswith(b'CAP ') and self.sasl and message[message.find(b' ', 4) + 1:] == b'ACK :sasl':
-			self.send(b'AUTHENTICATE EXTERNAL')
+
+		# SASL
+		elif message.startswith(b'CAP ') and self.sasl:
+			if message[message.find(b' ', 4) + 1:] == b'ACK :sasl':
+				self.send(b'AUTHENTICATE EXTERNAL')
+			else:
+				self.logger.error(f'Received unexpected CAP reply {message!r}, terminating connection')
+				self.transport.close()
 		elif message == b'AUTHENTICATE +':
 			self.send(b'AUTHENTICATE +')
 		elif message.startswith(b'903 '): # SASL auth successful
@@ -395,6 +403,35 @@ class IRCClientProtocol(asyncio.Protocol):
 		elif any(message.startswith(x) for x in (b'902 ', b'904 ', b'905 ', b'906 ', b'908 ')):
 			self.logger.error('SASL error, terminating connection')
 			self.transport.close()
+
+		# NICK errors
+		elif any(message.startswith(x) for x in (b'431 ', b'432 ', b'433 ', b'436 ')):
+			self.logger.error(f'Failed to set nickname: {message!r}, terminating connection')
+			self.transport.close()
+
+		# USER errors
+		elif any(message.startswith(x) for x in (b'461 ', b'462 ')):
+			self.logger.error(f'Failed to register: {message!r}, terminating connection')
+			self.transport.close()
+
+		# JOIN errors
+		elif any(message.startswith(x) for x in (b'405 ', b'471 ', b'473 ', b'474 ', b'475 ')):
+			self.logger.error(f'Failed to join channel: {message!r}, terminating connection')
+			self.transport.close()
+
+		# PART errors
+		elif message.startswith(b'442 '):
+			self.logger.error(f'Failed to part channel: {message!r}')
+
+		# JOIN/PART errors
+		elif message.startswith(b'403 '):
+			self.logger.error(f'Failed to join or part channel: {message!r}')
+
+		# PRIVMSG errors
+		elif any(message.startswith(x) for x in (b'401 ', b'404 ', b'407 ', b'411 ', b'412 ', b'413 ', b'414 ')):
+			self.logger.error(f'Failed to send message: {message!r}')
+
+		# Connection registration reply
 		elif message.startswith(b'001 '):
 			self.logger.info('IRC connection registered')
 			if self.sasl and not self.authenticated: