The same customer, three times

Phone numbers in four different formats. Names shouting in capitals. Trailing spaces that break your mail merge. Drop the list in and it tells you what's wrong before it touches anything — then shows you exactly which cells it changed.

When something is genuinely ambiguous — is 03/04 the 3rd of April or the 4th of March? — it asks instead of guessing. A cleaner that quietly moves your dates is worse than no cleaner at all.