I'm having trouble with a scheduling program

Basically, I've been working on a scheduling program for a while now, and it suddenly stopped working, and I don't know why. Here's what it does:

It stores the schedule items with their date and time in a file stored in /home/USER/.schedule/schedule.txt, as well as items that happen once a week in /home/USER/.schedule/recurring.txt. Here's the script that seems to be the problem:

for HOUR in {00..23..-1}
    do
        for MINUTE in {00..59..-1}
        do
            cat /home/"$USER"/.schedule/schedule.txt | grep -i "`date +%m/%d/%Y` $HOUR:$MINUTE" | cat /home/"$USER"/.schedule/recurring.txt | grep -i "`date +%A` $HOUR:$MINUTE"
        done
    done

I have no idea what's going wrong, or what I changed that broke it. Please help.

Edit 1: The problems seemed to start when I added a second and third user to the computer, though I have no idea why that would change anything.