I've mostly found that it's an opportunity for Murphy's Law to have a good go at me. Anytime that I forget my bottles the machine is working fine, no queue, usually not even being used and if there are more than one machine then both of them are working. Anytime that I remember my bottles there will be one machine working and one out of order/full. The working machine will have a queue of at least three in front of me. Of the people in front of me at least one person will either a) have a small child going at snail's pace helping or b) have a unbelievably huge number of bottles (yesterday the person in front of me returned over €11 worth of bottles) or c) not have rinsed the bottles, or put the lids on so that there's sticky soda/juice residue everywhere when I get to it.
On the positive side I have refined my technique and sped up to the point that I am positively a blur as I post the bottles into the machine. I have even issued enough threats at home that my DC remember to both put the lid back on and check whether the bottle is returnable and put it into the appropriate place. I now almost always remember to use the money back thingy at the till.