bitcoin-cli failed with error: incorrect rpcuser or rpcpassword When I run this command 

bitcoind -testnet -printtoconsole -rpcuser=123456 -rpcpassword=123456


After bitcoind is up. I run this on the same Ubuntu 17.10

bitcoin-cli -testnet -rpcuser=123456 -rpcpassword=123456 getbalance


I get this result:
error: incorrect rpcuser or rpcpassword (authorization failed)
The second day I tried exactly the same way. I get the correct result. It is really weird.

I try to put these two lines in /home/user/.bitcoin/bitcoin.conf

rpcuser=123456
rpcpassword=123456


And I run this command

bitcoind -testnet -printtoconsole -conf=/home/gogogo237/.bitcoin/bitcoin.conf


And this 

bitcoin-cli -testnet -rpcuser=123456 -rpcpassword=1234564 getbalance


notice that I intentionally changed -rpcpassword to a wrong password. But I still can get a correct result. 
This is really unpredictable and obviously a mistake. Anyone can explain this error? Thanks!
re-compiled on an Ubuntu system, fresh from github.com:

$ uname -a
Linux ubuntu-ae 4.10.0-19-generic

$ bitcoind -version
Bitcoin Core Daemon version v0.15.99.0-g96ac26e5662

$ bitcoind -testnet -printtoconsole -rpcuser=123456 -rpcpassword=123456

$ bitcoin-cli -testnet -rpcuser=123456 -rpcpassword=123456 getaccountaddress "myaccount"
n2r1aQiA37LLgy4qWjuLHXqA9ZhtcX4KG6

$ bitcoin-cli -testnet -rpcuser=123456 -rpcpassword=123456 getbalance
0.00000000


and with wrong password:

$ bitcoin-cli -testnet -rpcuser=123456 -rpcpassword=1234564 getbalance
error: incorrect rpcuser or rpcpassword (authorization failed)


so s.th. is wrong with the sources? Or with Ubuntu setup. What kernel do you have (uname -a) ?