Manually crafted multisig Tx: non-mandatory-script-verify-flag
I was sending an extra signature.

The transaction required 2-of-3, and I sent the 3 signatures. The extra signature was not popped off the stack by OP_CHECKMULTISIG, and the remaining stack looked like this:

EXTRA_SIGNATURE
OP_0 # null-dummy for Bitcoin Core's old bug


The extra signature took the place of the dummy, and bitcoind detected this when verifying the dummy was an OP_0.