The last place you want errors is in reference books, especially writing handbooks. But it happens, and you should be aware of it.
I noticed this error in the Fourth Canadian Edition of the Gregg Reference Manual. The preferred spelling entries on page xxxiv for words ending in –or or –our listed under the Gage Canadian Dictionary are incorrect. Standard Canadian English always uses –our (e.g., in honour, colour, favourite). A quick look at the Gage itself will confirm this: the first entry for all these words is the –our spelling. I don’t know if the error remains in more recent editions, but it’s still in the fourth edition, so Canadian writers (and especially editors of Canadian English) beware.
I was a little more surprised by the error in my brand new copy of The Canadian Oxford Guide to Writing. It’s the second edition too. A quick-reference block (p. 614) on perfect tenses reads like this (corrections in square brackets):
PRESENT PERFECT I shall have gone. [No, future perfect]
PAST PERFECT I have gone. [No, present perfect]
FUTURE PERFECT I had gone. [No, past perfect]
This is an ugly mistake, but the surrounding text is correct.
Now, I haven’t read the whole Oxford book either, so I don’t know if there are more errors. And the book looks like such a lovely and thorough piece of work. But it sure is an ugly mistake. It’s the sort of thing a copyeditor is supposed to fix—and one probably would have, if publishing houses hired them anymore.