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Canadian vending machines are happy to accept US quarters at par. Why pay for a more sensitive mechanism to prevent someone from giving you a more valuable coin. Although, probably not more valuable enough that it's worth filtering out from change boxes to deposit with a US based bank instead.

Otoh, there are coin mechanisms that can check for and reject quarter sized objects that are attracted to magnets; they might not be installed everywhere that takes US quarters, but if an operator starts getting a nuisance amount of CA quarters, they will be. See this doc [1] for a description of a pretty discerning mechanism (although it accepts both US and CA quarters, a slightly different design could reject CA quarters)

[1] http://segalandassociates.com/Documents/Coin-Mechanisms_Slug...