Nothing is 100% foolproof and as such so as to prevent inadvertant retraction an/or collapsing of gear when being towed gear safety pins must be installed. We also have a SOP when if the a/c is on the ground more than X hrs we have to put the pins in (reasons as detailed by fly4fud).
Although on our checksheets there is always a statement to check for removal of gear pins etc, if they have been installed I(we) always make an entry in the log that they are fitted, so that until somebody removes then stows them and clears the log the a/c can't effectively fly.
Of course nobody ever misses them on their walkaround do they!
A/C have taken off with them still in before and it will happen again!