Using 0.15.1 from BitcoinCore.org:

$ bitcoin-qt -rescan 1 -wallet otherWallet.dat 
PaymentServer::ipcSendCommandLine: Payment request file does not exist:  "1"
"PaymentServer::verifySize: Payment request too large (720896 bytes, allowed 50000 bytes)."

Maybe this pilot error could be handled better. For example, Bitcoin-Qt (or bitcoind) could refuse to start.

Note that in the above case Bitcoin-Qt silently ends up using a wrong (default) wallet.