Yes, it matters a lot.
When you are separating egg whites, it is for whipping them into a foam.
This foam is a protein-based foam, relying on protein ends hooking into each other.
Even small traces of fat will prevent the foam from forming.
Egg yolks contain high amounts of fat.
Once an egg yolk breaks in your whites, you have to start the separation anew, because it can prevent your foam from forming.
Also, don't use plastic bowls for whipping egg whites (their surface retains some fat molecules even after washing, giving you a less stable foam) and only whip with a cleanly washed whisk or mixer attachment (not one you have just used for something else, not even if you wiped it clean).
To prevent big mishaps, just separate each new egg in a teacup and only add the new white to the old whites after it has separated cleanly.
Else you are in big trouble if you are separating a lot and the last egg contaminates the whole whites with yolk.
And a single contaminated egg is easily reused for a quick egg-and-feta sandwich or something else.
Contamination
the other way round isn't so worrisome.
You still want to work as precisely as possible, as yolk-only recipes will often have somewhat worse texture if eggwhite is included, but small contaminating amounts are usually not noticeable in the finished product, even in foams (zabaglione, mayonnaise).
This is because yolk foams are fat-based, and small amounts of protein don't prevent a fat foam from forming.
cooking.stackexchange.com/users/4638/rumtscho
I just stumbled upon this to see if I ruined my angel food cake when some egg yolk leaked into my whites.
I spooned out as much as I could
but there was still a little in the whites
but I didn't have enough eggs to start over.
Gave it a go, and I was able to get stiff peaks.
Took a tad longer than normal
but I got stiff peaks nonetheless.
cooking.stackexchange.com/users/34382/Lauren
I know that this question was from over a year ago, but id just like to point out that i make meringues at least once a month and often a little bit of yolk gets in.
This is never a big issue!!I
just whisk the egg whites like normal and
almost always it is fine.
Good Luck with future meringues!!
cooking.stackexchange.com/users/22298/Meringuemaster
I had the same problem this morning as I was trying to make waffles, but I still got stiff peaks.
I had a significant amount of yolk in my whites and
what I did was I tried to scoop as much yolk as I could out of the whites with a spoon and, even though there were still some wisps of yolk leftover, the whites still became stiff peaks quite quickly (of course I was using the second highest setting on my electric mixer).
Waffles were delicious!
cooking.stackexchange.com/users/22850/Genny
Before discarding, you can give this a try.
Worked for me today (I was making tiramisu).
Lift as much of yellow specks from the egg whites in your bowl as you can and then start beating it, adding little sugar at a time.
Even after beating long enough if you do not see the anti-gravity feature of the meringue, just let your bowl sit undisturbed for some time (maybe half an hour).
Once you are back, you should see a foam-like surface on the bowl.
Scoop the foam up carefully with a spoon
(don't dig too deep, else the liquid might come in too) and now hold the spoon upside down.
If the foam doesn't fall off, yippee, there you have your meringue!!
Collect as much of this foam as you can(remember to check for the anti-gravity feature) and add it to your cream mixture (in case of tiramisu), very gently mixing it in.
You will find that the cream slowly thickens.
Ofcourse, this method will not produce as much meringue as expected out of x number of eggs, but, it does come in handy.
cooking.stackexchange.com/users/20005/indianabroad
Well I have had a little contamination from a broken yolk on two occasions but both times pressed on and had no problem getting the mix stiff enough in a normal timescale and the end product was perfect.
I was making pavlova.
So in my mind I have disproved the old wives tale that even minute amounts of yolk contamination make a foam impossible to sustain.
I think the sugar beaten in stabilizes it strongly.
cooking.stackexchange.com/users/61414/Martin
A speck of yolk shouldn't matter but more than that you should start over because you'll be disappointed in the final result I speak from decades of baking
cooking.stackexchange.com/users/48416/Patty of Wales
If you are going to beat the egg whites to peaks, a teeny tiny bit of yolk wont spoil the effort IF you add 1/4 tsp of cream of tartar.
This is my experience.
cooking.stackexchange.com/users/68856/manu muraleedharan
I was separating egg whites today and one of the yolks broke and contaminated my bowl of whites. I started from scratch, but I'm wondering if I really needed to; would a tiny bit of yolk (say, 1/8 tspn in 4 egg whites) mixed in with the whites make a big difference?  To provide some context, I was about to whip the whites with castor sugar to make almond macaroons.
When I'm separating egg whites, how much does it matter if a little yolk gets in?
cooking.stackexchange.com
<eggs><egg-whites>
cooking.stackexchange.com/questions/23727
cooking.stackexchange.com/users/6767/Highly Irregular
Even a small amount of yolk can impede the foaming process, but adding 1/4 tsp of cream of tartar in egg whites with a tiny bit of yolk will probably ensure that they can be beaten to peaks. In theory, any contamination will prevent the eggs from forming peaks. However, it is possible, especially if you add a bit of cream of tartar.
Any contamination will prevent the eggs from forming stiff peaks. You can still get stiff peaks, especially if you add a bit of cream of tartar.