What single word can explain what's wrong with the following phrases?

PET PAIL

PEA PLUG

PAD PACK

PADDLE PORE

PEER PUCKER

Answer:

 

STOP - - - parse as three words: 

S (has been changed) TO P

 

SET SAIL

SEA SLUG

SAD SACK

SADDLE SORE

SEER SUCKER

 

Credit:

Paul Coulter