Knitting is often a process and in all processes mistakes happen. That the process also can turn into a usable item is one of the many reasons why mistakes must be fixed. As is frequently the case, there is a continuum sometimes mistakes can be ignored, but sometimes they have to be fixed. When a person has painstakingly hand knitted a garment, even something as small as an afghan square, they generally want to ignore mistakes if possible. A knitted garment may have as few as 90,000 stitches or as many as 180,000 stitches. For example, if you are knitting an afghan square and it has 40 stitches in the row and you are on row eight and notice a mistake in row two should you rip back, start over, or just keep going? You have already knit 320 stitches, but if you are going to make a baby afghan that is five squares wide by five squares long you will have to knit 60,000 stitches in total (40 stitches in a row, by 60 rows, times 25 squares). Is it worth continuing on and ignoring the mistake?
Again, as is frequently the case, it depends on the knitter.
If the mistake is minor and only you notice it you might want to ignore it. If the mistake is changing your stitch count and you constantly have to adjust your pattern, you might want to fix it or start over. There are times when a mistake isn’t glaring, but because you are learning a new technique, don’t have that much time invested yet, or the mistake will cause problems down the road you will also want to rip back or start over. If the mistake can be called a “design element” or will not affect anything including your enjoyment of the item then you may want to just ignore it.
The biggest piece of advice when deciding to rip out a mistake or ignore it is never do it when you are frustrated, or tired. Wait a night or day or two before deciding. A “glaring mistake” at midnight may not be noticeable in the morning and so it can be easily ignored. A minor mistake may make you not want to pick up the piece again and therefore rather than waste the yarn rip it out and start again. Most of us have left in mistakes that in retrospect we wish we hadn’t and at other times felt we wasted a lot of effort correcting something that wasn’t worth correcting. There really is no hard and fast rule, it truly does depend on the individual. Someone close to me uses the 20 mile an hour rule, if you were driving by at 20 miles an hour could you see the mistake, if yes-fix it, if no-don't!