Suddenly I get "Error:checksum mismatch, Last Line: 30" and other errors

Sorry for the long topic title.
Anyway, here goes:

All of a sudden, today for the first time, I get weird errors during printing, and here's an error I got during PID configuration:

The line I sent was: M303 S245
Worked well, but after finishing this happened:

< 10:22:19 PM: PID Autotune finished ! Place the Kp, Ki and Kd constants in the configuration.h
< 10:22:19 PM: Error:checksum mismatch, Last Line: 30
< 10:22:19 PM: Resend: 31
< 10:22:19 PM: Error:Line Number is not Last Line Number+1, Last Line: 30
< 10:22:19 PM: Resend: 31
< 10:22:21 PM: Error:Line Number is not Last Line Number+1, Last Line: 30
< 10:22:21 PM: Resend: 31
< 10:22:22 PM: Error:Line Number is not Last Line Number+1, Last Line: 30
< 10:22:22 PM: Resend: 31
< 10:22:23 PM: Error:Line Number is not Last Line Number+1, Last Line: 30
< 10:22:23 PM: Resend: 31
< 10:22:24 PM: Error:Line Number is not Last Line Number+1, Last Line: 30
< 10:22:24 PM: Resend: 31
< 10:22:25 PM: Error:Line Number is not Last Line Number+1, Last Line: 30
< 10:22:25 PM: Resend: 31
  10:22:25 PM: Connection closed

I have run one PID autotune round before, and this happened:

< 9:58:57 PM: PID Autotune finished ! Place the Kp, Ki and Kd constants in the configuration.h
< 9:58:57 PM: Error:checksum mismatch, Last Line: 128
< 9:58:57 PM: Resend: 129
< 9:58:58 PM: Error:Line Number is not Last Line Number+1, Last Line: 128
< 9:58:58 PM: Resend: 129
< 9:58:59 PM: Error:Line Number is not Last Line Number+1, Last Line: 128
< 9:58:59 PM: Resend: 129
< 9:59:00 PM: Error:Line Number is not Last Line Number+1, Last Line: 128
< 9:59:00 PM: Resend: 129
< 9:59:01 PM: Error:Line Number is not Last Line Number+1, Last Line: 128
< 9:59:01 PM: Resend: 129

Etc etc, it just keeps on looping until I disconnect the printer.

Anyone knows why this happens all of a sudden?
I didn't change anything!

I don't what software you are using, but I found a funny thing, both the host software I was using (pronterface) and the firmware (teacup variant) had bugs with resending lines. In the case of a checksum error, I got a similar error loop to yours. The error messages indicate that both ends are expecting the same next line, but in fact it gets rejected!

It seems that error handling was never working properly, perhaps it had at one time but suffered from bit rot. The checksum method is not very robust anyway, e.g. if the "*" is dropped, then the checksum is not checked and the line not rejected. I think the only way the comms works reliably is if there are no comms errors. I see people often report problems like "printer hangs during print", for example.

What software are you using? If I can dig out my changes I might be able to suggest a patch.

Ah yes, I am using Repetier Host with the latest Marlin Firmware on MAC through USB.

I also had the problem today with one print where I had Slic3r generate forced support for the first 12 layers of a print from within Repetier Host. When I removed the forced support layers, it worked without any problem. With the support on it crapped out on me around layer 2 or 3 and started shifting layers all over the printbed.

any reboots then testing again? do you have another usb cable that you can try? any new software installation, or system updates? things to check besides firmware...