One common spelling error is omitting the K at the beginning of words where it is silent (example: nick-nack instead of knick-knack). The origins of silent K are difficult to pin down. What we do know is that the k wasn’t always silent, especially in words of Germanic origin. Just as it is in German, the k was actually pronounced and many of the words which now have silent k originally began with that distinctive clicking sound. My high school teacher made a point of this when teaching us Chaucer.
However, some time after the Chaucerian era, the k sound disappeared from the kn combination. There is speculation that it suddenly became hard to pronounce, and was just one of many linguistic changes after the Great Vowel Shift. I haven’t found any corroboration for this theory.
Here’s a partial list of words with a silent K. Feel free to add yours.