When I search on Google and use
+ to indicate that a word must be on the page I'm searching, why do I get a bunch of results without that word?
It was first noticed in October, 2011.
They changed that behavior shortly before Google+ launched. The idea is that the plus would indicate G+ content. This is borne out by the search operators help page. (Oddly enough, it's also used for blood types.)
The new way to "force" a word to be in the results is to enclose it in quotes.
When you put a word or phrase in quotes, the results will only include pages with the same words in the same order as the ones inside the quotes. Only use this if you're looking for an exact word or phrase, otherwise you'll exclude many helpful results by mistake.
Example: "imagine all the people"