When trying to sign a transaction in a encrypted wallet that is locked, version 0.16 would return the following error:
"Please enter the wallet passphrase with walletpassphrase first."
While doing exactly the same procedure against 0.17, the response is something like:
 0"hex": "020000000183ac9cc1137c444f32356fb070ecba194c87a05e43674c06a55f5b1d0561eabc0000000000ffffffff0180626a94000000001600141875aa6ab19c2551c11759cf3e9f4dd7aa40bcf500000000",
 1       "complete": false,
 2       "errors": [
 3         {
 4           "txid": "bcea61051d5b5fa5064c67435ea0874c19baec70b06f35324f447c13c19cac83",
 5           "vout": 0,
 6           "witness": [
 7           ],
 8           "scriptSig": "",
 9           "sequence": 4294967295,
10           "error": "Unable to sign input, invalid stack size (possibly missing key)"
11         }
12     ]
v0.17 should check if the wallet is locked before trying to sign the tx and return the same error as v0.16 for consistency reasons.