The most frequently encountered octet “violators” are atoms with three valence electrons. Neutral boron or aluminum in compounds like or have only 6 in the valence shell: 3 from the B or Al and another three from the three covalent bonds. These compounds can acquire a filled octet (ex. or ), but not without becoming charged.
The other frequently encountered electron-poor atom is carbon in a carbocation. These tend to be highly reactive compounds that readily accept an electron, making them powerful electrophiles.