Everyone accidentally loses count from time to time. You're in the middle of a long workout, there's no oxygen getting to your brain, your heart is beating out of your chest and as you pick up the bar, kettlebell or medicine ball you think "was I at 47 or 57?" Your willingness to take the lower number and do the possible extra reps says great things about your character. How you do anything is how you do everything, right?
Another issue entirely is purposefully shaving reps, doing less than you've commited to do prior to the workout, or just moving onto the next movement or round when someone else does or you get tired. In a three round workout of 10 Clean and Jerk, 15 Burpees, 20 Wall Balls, an athlete might do 9 Clean and Jerks one round, 7 the next, and 8 on the last round. Burpees go 12, 10, 10, and wall balls are 20, 17, 16. At the end of it all the athlete says "my time is 8:42! I felt pretty good today!" Scaling the reps prior to the workout to 10 C&J, 10 Burpees and 15 Wall Balls is totally acceptable and necessary at times, but shaving reps and doing random numbers of reps is not!
I think this happens for multiple reasons. Not wanting to be the last one done and not being mentally strong enough to keep pushing when the workout gets tough and you fall behind are possible reasons in addition to flat out cheating to win. If one of the first two reasons apply, SCALE THE WORKOUT from the beginning and do all your scaled reps. If you're just cheating to win, DON'T BE THAT GUY/GIRL. In either case, it may interest you to know that Coaches sometimes count your reps, and so do other athletes. If you're not doing what you say you are, or logging a different result on Beyond the Whiteboard, chances are that your Coach knows you're BS'ing and so do your friends! Remember, the main reason we track results is to track progress. If you're entering phony info, then you have no real results to compare, and it will be impossible to see yourself progress. You're only fooling and hurting yourself!
Coaches are there to help you pick the appropriate amount of work for each WOD. Ask them for help! If you just need to learn to count, get a math tutor or ask a 1st grader!