Reports from the UK show that authorities have made more than 500,000 requests for confidential communications data over the past year alone - roughly 1 for every 78 adults in the UK. These requests come under an Act that allows authorities to gain access to information on who individuals have been phoning, emailing, which websites individuals visit, etc.
Originally enacted for the purpose of combating terrorism, the UK example shows how easy it is for lawful access legislation to lead to abuse - in some cases, the access powers were used to snoop on suspected litterers.
Defenders of the UK Act, like those who tout the Canadian version, say the Act does not allow access to the content of communications, but only the 'traffic' (who you are speaking to, etc.).
While the Canadian version of lawful access is not as broad as the UK version, this demonstrates that it is indeed a slippery slope and care should be taken to ensure the proper safeguards are in place before any expansion of police powers is allowed.
See the UK Information Commissioner's full report as well as a breakdown of some of the numbers.