If you use NFS to mount your root filesystem you should probably faced with this problem:

Root-NFS: Unable to get mountd port number from server, using default
Root-NFS: Server returned error -5 while mounting /my/nfs/server/path/
VFS: Unable to mount root fs via NFS, trying floppy.

The -5 and -13 are the most common on my daily usage, but I always forgot what this number means… So, I decided to get it from source by checking nfs-utils package. The following values where extracted  from utils/mount/error.c.

EPERM  ........................................ -1

ENOENT ........................................ -2

EIO ........................................... -3

ENXIO ......................................... -4

EACCESS ....................................... -5 

EEXIST ........................................ -6

ENODEV ........................................ -7

ENOTDIR ....................................... -8

EISDIR ........................................ -9

EINVAL ........................................ -10

EFBIG ......................................... -11

ENOSPC  ....................................... -12

EROFS ......................................... -13

ENAMETOOLONG .................................. -14

ENOTEMPTY ..................................... -15

EDQUOT ........................................ -16

ESTALE ........................................ -17

EWFLUSH ....................................... -18

Knowing what each error status means allow you quick fix the problems.