what would happen?

If you pitched a lager and ale yeast together. And left it at 65 degrees. i know you'd get beer, but will that create off flavors?

Short answer: probably. The lager yeast should ferment in the 50s. At that high of a temp, the ale yeast would be fine. The lager yeast will produce some less-than-desirable flavors that will probably ruin anything that the ale yeast did to produce good beer. As an experiment, you could try it and see what happened. However, I wouldn't do it and expect great beer to come out.

EDIT: I should have noted that some yeast strains are blends of lager and ale yeast, so if you're using one of those in the type of beer for which it is intended, then you'll probably turn out a great beer provided, of course, that the rest of your process is solid, too. If you're just mixing two random types of yeast, then you're outcome might not be so great.

At any rate, have fun with it.

My short answer; hard to know. Most likely is that one yeast is going to take off more quickly than the other and dominate the fermentation. If that's the ale yeast, you'll basically have created a normal ale, maybe with a few esters from the lager yeast fermenting too warm. If the lager yeast for some reason takes off, more likely to end up with more esters.

I suspect in the end, you'd have something not unlike a California Common.