I am creating an image of 150mm x 100mm at 300dpi but photoshop keeps adding decimal points like 150.02 when i change the image from the orinal size, is it something I am doing wrong or a known bug? You would expect that when you change an image to 150 x 100mm it would stay at that size and not be added to.
You are using inches for the resolution (300 "dpi"—you mean 300 ppi) and mm for the image dimensions.
You can't split pixels - you must always have a whole number of pixels after any resampling operation. That's the root of the problem. Think about it.
My general advice: Think about the pixels when editing and worry about setting the physical size when it comes time to produce your work products. If it takes 342.6714 pixels per inch to get the editing result to exactly fit the paper at the size you want it, so be it.