The Liberals, however, suggest the incident is a "deliberate attempt at voter suppression" and have filed a complaint with Elections Ontario calling for an investigation.
Deputy premier Deb Matthews, who has held the riding since 2003, intercepted reporters outside Tory Leader Tim Hudak's event in London North Centre, saying two Liberal supporters in the city had received the letters.
The local Tory candidate, Nancy Branscombe, then issued a statement apologizing for the "confusion" the letters from her campaign office may have caused to residents of 64 households.
An aide said a campaign volunteer put letters meant for Tory supporters in the wrong envelopes.
But the Liberals later said similar notices were sent to voters in another contested riding, Ottawa West-Nepean.
In a second message to Elections Ontario, lawyers representing the Liberals ask the agency to "take immediate measures" to ensure "any misapprehension" about voting locations is cleared up before the vote.
"At this point, we do not know how widespread the distribution of this misleading letter is," they write, adding "it now appears there could be a pattern of conduct aimed at voter suppression."
The Tories later issued a statement apologizing for the inconvenience caused to a "small number" of Ottawa residents.