Say you replace just the word "images" with "C://project/images" it could also replace the word in, say, a textfile that just happened to use it. Or a function in a script has "images" as part of the name, replacing that obviously breaks the script.
It's less likely to happen if you do it by hand, but in an automated process it can happen very easily. So if you automate it you'll just have to be careful to pick the right strings to replace and possibly do it file by file rather than all files at once/recursive.