In a similar vein, inodes can run out. On most conventional Linux file systems, inode numbers are 32 bits.

For many, this is not going to be a practical problem yet, as real volumes will run out of usable space before exhausting 2^32 inodes. However, it is theoretically possible with a volume as small as ~18 TiB (using 16 TiB for 2^32 4096-byte or smaller files, 1-2 TiB for 2^32 256- or 512-byte inodes, plus file system overheads).

Anticipating this problem, most newer file systems use 64-bit inode numbers, and some older ones have been retrofitted (e.g. inode64 option in XFS). I don't think ext4 is one of them, though.