Have you ever seen tzitzit with one string a lot shorter than the rest? I certainly have. Here's an inquiry we received today from a customer who had tzitzit tied locally, and was disgruntled with the results he got.

I recently had my tzitzit replaced on my Talit Gadol after my local dry cleaning shop somehow destroyed them. After receiving my newly updated Tallit, I noticed there’s one string of the 8 that is significantly shorter than the others on all 4 kanfot. I’m not sure why this happened but it’s greatly disturbing me as I would have preferred that they were all at equal length or at least close to equal. Can you tell me if there is a reason why this occurred? Is this halachically permissible or do I need to purchase a new Tallit? I’ve attached a photo for your reference [scroll down to bottom]. Thanks, Moe J.

Here's my reply to Moe:

Sorry to hear about the dry-cleaning mishap.

Well, it's a bit hard to tell for sure from the picture [below]. Sometimes a tzitzit tie-er doesn't wind around very tight, so he uses up more of the length of the shamash strings (when you start out on each corner, you use three regular length strings and one long string). Also, different string manufacturers have different lengths. Some are not as generous and don't really give you much length in the shamash string; an experienced tie-er will anticipate the problem from the start – or when at least by the time he's got it halfway tied – and will switch to another string to accommodate the shortage. But if you're oblivious and just tie away, when you finish up suddenly you see that your shamash string didn't go the distance.

(By the way, you should tidy up those last double knots I'm seeing in the photo you sent. One looks like a single knot, another is also a single knot which is already loosening up and a third has not double knot at all. Before you tie up those knots, say: Leshem mitzvat tzitzit, which is a declaration of intention that you're doing the tying to fulfill the mitzvah.)

One short string certainly does not render the tzitzit not kosher. In fact, it might not even be considered "short" from a halachic standpoint. According to the more lenient opinion, 24 cm from the start of the first double knot down to the tips of the strings is long enough.

So if that one string gets past the 24 cm mark, you're fine. If not, you're still good to go because you have three intact strings, which is enough to be considered kosher.

Even if technically the length is okay, what about the fact that it looks skewed to have one short string? Indeed, this is mentioned in the Gemara.

ונוי תכלת שליש גדיל ושני שלישי ענף

That means the loose part should be twice the length of the tied part, to fulfill the mitzvah in an aesthetic manner. If your Hebrew is solid and you'd like to delve into the matter, there's a good article here, and more rudimentary articles in English here and here.