1) You have some small amount of feathering set as a default when you make a selection. Check the Options bar just after making a selection to see if there is a feather amount, if there is, change it to zero.

2) It's anti-aliasing. If this is the case, you can turn off anti-aliasing in the Options bar, right next to the feather amount when you have a selection tool active.

If it's neither one of those things, then you have a demon in your computer. Sacrafice a chicken, do a rain dance, quit Photoshop and hold Command + Option + Shift while restarting Photoshop (to delete the prefs).

Hey JimD and thanks for the reply.

Points 1 and 2 was the first thing I checked.

I tryed to start Photoshop with holding Command + Option + Shift but still the same.

Maybe reinstall is my only choice now :/ ?

Life is strange and so is Phoroshop.... :)

after holding down those keys, you should get a dialog that asks if u want to reset the pref file. if you did not get that dialog, then you need to try again only this time press the keys as fast as possible right after you clicked the psp icon.

also, im thinking, go to preferences > general and look for odd settings under the image interpolation. maybe u got bicubic smoother there, or something. toy around a bit.

i usually flatten a copy of the image entirely, i do not trust merging.
i sometimes find that it gives me strange results.
and the guides and slices preserve if you flatten.

also, one thing. if u drag the guides from the ruler, they might end up at non integer coordinates. that gives you problem with px accurate selections.

so, the best way is either to go by numbers View > New guide and type in what you want.
or,
my preferred method is to drag a marquee, and then drag and snap a new guide to the marquee. the marquee lands always on integer coordinates. so will the new guide. do this for every guide and your life will be smoother.

i hope it helps, pls let us know

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always outnumbered, never outgunned