I went down to the HRDC office nearest my house this morning to file a form.
About five years ago, I worked in Canada for the summer. As a non-resident foreigner, I should not have been paying Canadian employment taxes -- Canada Pension, for example, and Canadian unemployment "insurance" (called "Employment Insurance" here). But these taxes were withheld from my salary.
I discovered this the next year while filing my U.S. income taxes. I tried to get the payments refunded by the government of Canada, but they wouldn't talk to me unless I had a Social Insurance Number. I applied for an SIN, but was told that I couldn't get one since I didn't have a currently valid work authorization.
So I filed my U.S. taxes and claimed the Canadian taxes as a foreign tax credit. You can't normally claim a tax credit on refundable payments, but my rationale (which the IRS accepted) was that at present I didn't believe I could get the money back from the Canadians. If they ever gave me the money back, I told the IRS, I would amend my return.
After another year or so of corresponding with Revenue Canada, they sent me some tax forms that they had filed for me and a check refunding the tax payments, with a little interest. I adjusted my old 1040 and paid a little extra tax, and a little interest. I came out ahead on the whole deal -- about $100, I think.
But the important thing was that Revenue Canada had assigned me a Social Insurance Number when they sent me the check. When we moved up here in August '01, I used that SIN on bank forms, income tax forms, credit checks, everything that requires an SIN. People were weird about it, because it starts with a 0, but it checksums correctly.
Today I was told that it's not a valid SIN and it will take three weeks to assign me a new one.