Fake Irish: A borderline example with the Connor family. While Michelle, Liam and Paul were presumably reared in England, they were born to Irish parents and seem to have kept close contact with Ireland (their parents returned to Dublin to live). Of the three, only the actor playing Paul had any known Irish ancestry and even his parents were English-born.
From the current cast, Craig Charles of Red Dwarf fame plays Lloyd Mullaney; Keith Duffy from Boyzone plays Kiaran McCarthy; Alison King (Carla Conner) used to star in Dream Team on Sky One; Ben Price (Nick Tilsley) used to have roles in Footballer's Wives & Casualty; Kym Marsh from Hear'Say plays Michelle Conner; Alex Bain (Simon Barlow) used to be in adverts for Rice Krispies; and Chris Fountain took over the role of Tommy Duckworth in early 2011.
Plus, there's the list of people who have made brief guest appearances, which includes Sir Ian McKellen, Peter Kay & Prince Charles.
Cherylee Houston, who plays Izzy Armstrong, previously appeared on a couple of episodes of Little Britain.
Sinead is played by Katie McGlynn, who played Scout on Waterloo Road.
Mace Tyrell was the doctor that was treating Tina's head injuries in May/June 2014.
Tracy Barlow. Played by various actors, with Dawn Acton and Kate Ford being one of the best-known.
Tracy's step-brother Peter has been played by six different actors. One even reappeared recently as a completely different character (Kirsty's father).
Most of the characters introduced as children.
Nick Tilsley. Twice, and both times as an adult.
Averted with Chesney Brown — Sam Aston has played the character from a child to an adult.
Post Script Season: Probably the ultimate example — it has essentially been on a series of postscript seasons since 1961. It was originally intended to be thirteen episodes long, with Coronation Street bulldozed in a Made-for-TV Movie. However, it proved so popular that a new series was commissioned, and it's been broadcasting more or less continuously to the present day.
Averted with William Roache (Ken Barlow) & Michael LeVell (Kevin Webster) - both characters were written off the show whilst their actors were awaiting trial for sexual offences. Both actors were found not guilty, allowing the show to bring them back to the show.
Chris Fountain (Tommy Duckworth) was abruptly fired after a homemade rap video where he sang about rape was discovered online.