As others have noted, this usually happens because both sides wrote data and one side didn't read it before calling close().

Here's a little reproducer: https://gist.github.com/jcalvinowens/da57edda9a01ca9f4c4088a...

    $ gcc -O2 test.c -o test
    
    $ strace -e socket,connect,write,accept,read,close ./test --rx        
    <...>                                                                           
    socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 3                                                                           
    accept(3, NULL, NULL)                   = 4                                                                            
    close(3)                                = 0                                                                            
    read(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4096
    <...>
    read(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4096
    read(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 3035
    read(4, "", 4096)                       = 0
    close(4)                                = 0
    +++ exited with 0 +++

    $ strace -e socket,connect,write,accept,read,close ./test --tx
    <...>
    socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 3
    connect(3, {sa_family=AF_INET, sin_port=htons(31337), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
    write(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 600000) = 600000
    close(3)                                = 0
    +++ exited with 0 +++

...versus:

    $ gcc -O2 -DWRITE_TO_SOCKET_BEFORE_READ test.c -o test
    
    $ strace -e socket,connect,write,accept,read,close ./test --rx
    <...>
    socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 3
    accept(3, NULL, NULL)                   = 4
    close(3)                                = 0
    write(4, "\250\3\0\0\0\0\0\0\250\3\0\0\0\0\0\0$\0\0\0\0\0\0\0$\0\0\0\0\0\0\0"..., 4096) = 4096
    read(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 4096
    <...>
    read(4, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 4096) = 997
    read(4, 0x7ffd45c2d3c0, 4096)           = -1 ECONNRESET (Connection reset by peer)
    <...>
    +++ exited with 1 +++
    
    $ strace -e socket,connect,write,accept,read,close ./test --tx
    <...>
    socket(AF_INET, SOCK_STREAM, IPPROTO_IP) = 3
    connect(3, {sa_family=AF_INET, sin_port=htons(31337), sin_addr=inet_addr("127.0.0.1")}, 16) = 0
    write(3, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 600000) = 600000
    close(3) 
    +++ exited with 0 +++

The occasional ECONNRESET