A p4 check-in failed with the following error:
Submit aborted -- fix problems then use \'p4 submit -c XXXX\'.
Some file(s) could not be transferred from client.
I had to submit a changeset recently that had 400,000+ files, and it failed with the error:
Submit aborted -- fix problems then use 'p4 submit -c 11111'.
There was not enough history in the console buffer to review what failed and what succeeded. So I ran the command suggested in the error message and output stdout to a file:
p4 submit -c 11111 > p4_submit_11111.txt
When this ran, errors (stderr) was output to the commandline, allowing me to resolve the problems (as described in other answers). In my case, it was a file I'd added to the changeset but since deleted, as per the first point in Mark's answer:
open for read: /home/myUserName/yada/_files.txt: No such file or directory
So to deal with this, I just reverted it from the changeset...
$ p4 revert /home/myUserName/yada/_files.txt
//P4Root/Proj1/yada/_files.txt#none - was add, unlocked and abandoned
... and then re-submitted the changelist using the same command:
p4 submit -c 11111 > p4_submit_11111.txt
This submit succeeded.