Thief is due to the press without alignment or set enough time where color piece together with a white line without ink. Due to many printing plant is a manual alignment, the thief is hard to avoid. As well as the relationship between color, if the C, M the two color color piece together, printing when the thief was particularly obvious. Generally in adjacent color piece with a different version of color, like the left one with M, Y of two color version of a red, while the right side only C color version and none of the M, Y the composition of the blue, this kind of circumstance to appear the thief.
A thief, is between the two color piece with an edge on both sides of the color of very fine, if take the example of the above, it is written in C, M, Y to do the edge.
And general black text can do trap (dieyin), and can't do in PHOTOSHOP is not do make up for the thief (except special requirement).
First, printing with CMYK4 color is a hit a color version, because the four colors are artificial set out, so the hard to avoid some not, if two adjacent color piece, with a different version of color, that there may be no alignment, and cause no ink, the do very exaggerated, basic is the mean.
High-grade printing leakage white is very exquisite, but not for the limitation of general design work is not too much, although according to your idea to design.