I was just making a sims and noticed this weird seam on his leg, not sure why its doing this.

It's probably your custom skin.

Nope tried taking it out and its still got the seam just not as bad.

Actually I think its is the skin, but all skins seem to do that for me or at least 3 different ones.

Is your Sim wearing a piece of CC as a bottom or outfit? It could be a case of this: http://modthesims.info/d/479479

Nope I dont have cc clothing, I also have this seam on the neck too :L Though that seam is only on my skin, not the ea one.

The answer is there!

Also, I'm pretty sure that skins and their effects can still show up if you haven't cleared your cache files since deleting them.

Looks like a clean patch where he wee'd down his leg!

In my experiences, EA's skins always have this weird seam across the legs and neck (Or well, where the sculpt mesh starts). Like not too obvious but if you'd really focus on it you'll see it! ^-^ If it'd have been a CC related issue (Except for skins) You'd see a seam across the ankles as well, which is not the case here. So I guess it's just a matter of- like orose suggested- to clear your caches, though to me it looks pretty 'normal' to me! ^-^

A lot, I would say most, skins have subtle seams on the legs, where the shoulders/top of the arms meet, where the face meets the scalp and sometimes also where the head meets the neck. Annoyingly, the EA default skin is not even seamless - if you look closely, you'll notice that there is a noticeable different between the front/back on the left side of the sim's abdomen. Sometimes you can have seams caused by mismatched normals on a CC mesh, but that doesn't look quite like your screenshot, as Greenplumbob stated. The only way to fix the seam in your screenshot would be to correct it on the skin itself, or switch the skin for one that is seamless or at least has a less noticeable seam on the leg.

Since Sims 2, I don't see those seams on EA skins, but on 80% of the CC skins. It's generally a seam on the upper part leg of the leg (inside part), seams on the ankles or even the shoulders are rare, thankfully. I always wondered why the skin authors don't see them and came to the conclusions that they must be using some HQ res mods and options to max so that there's so much noise and contrast everywhere that they miss those seams lol, like a tree in the forest.
I indeed remember seeing a navel 'disconnected' in an EA skin (i.e. split in half and the halves not correctly connected) - I think it's in Sims 3 but not sure since I always used a CC skin. Maybe YAM.
You could edit the skin yourself but it's tedious and very time consuming. Not hard at all, but it's a long boring process. (now maybe that's why the authors don't see them...:p)