There are always problems like this that surface if you make changes to geriatric code. Quite often they are minor with just parameters needing to be increased because the system is now doing extra work or has more interfaces so time out failure needs to be longer or a count needs to allow a higher value. When you are down coding at machine code or if you are lucky at assembler level and you are working on the physical machine these small things can bite you. Had the original designer still been around you might have been warned don't change that value without altering this other - apparently unrelated- parameter. The problem with maintaining embedded code that was written before ideas of structure and to fit inside the space/run-time available is that whatever you change may cause an error somewhere. Maintenance programming at machine level is not a skill that is taught any more.