It also seems quite easy to verify this hypothesis with scapy:

  >>> p1 = IP(dst="192.168.mac.ip")/TCP(dport=1984,sport=20001)
  >>> p2 = IP(dst="192.168.mac.ip")/TCP(dport=1984,sport=20002)
  >>> p2.show2()
  ###[ IP ]###
    version   = 4
    ihl       = 5
    tos       = 0x0
    len       = 40
    id        = 1
    flags     =
    frag      = 0
    ttl       = 64
    proto     = tcp
    chksum    = 0xf905
    src       = 192.168.linux.ip
    dst       = 192.168.mac.ip
    \options   \
  ###[ TCP ]###
       sport     = commtact_http
       dport     = bb
       seq       = 0
       ack       = 0
       dataofs   = 5
       reserved  = 0
       flags     = S
       window    = 8192
       chksum    = 0xb836
       urgptr    = 0
       options   = []

  >>> p2[TCP].chksum = 0xb836 ^ 0x8  # mangled checksum

  >>> sr1(p1, timeout=1)
  Begin emission:
  Finished sending 1 packets.
  ....*
  Received 5 packets, got 1 answers, remaining 0 packets
  <IP  version=4 ihl=5 tos=0x0 len=44 id=0 flags=DF frag=0 ttl=64 proto=tcp chksum=0xb902 src=192.168.mac.ip dst=192.168.linux.ip |<TCP  sport=bb dport=microsan seq=1671494800 ack=1 dataofs=6 reserved=0 flags=SA window=65535 chksum=0x6039 urgptr=0 options=[('MSS', 1460)] |>>

  >>> sr1(p2, timeout=1)
  Begin emission:
  Finished sending 1 packets.
  .....................................................................................................................................................................................
  .....................................................................................................................................................................................
  ....................................................................................................................................
  Received 494 packets, got 0 answers, remaining 1 packets

My Mac silently dropped the packet with the mangled checksum.