"U is for Una who slipped down a drain"
Una was just having a normal day, taking a small stroll on a path that she knew so well, that she could walk it blindfolded and backwards. Going this certain way, was just the norm to her, and anyone else who was familiar to see her coming by. This path was just part of her daily moments.
Nothing special for that, nothing out of the average passing time to drift away for her, Una was just having her day like any other. Maybe on some days, Una would do something strange and take her stroll with her eyes closed, or give it a try now and then to walk it backwards. If anything; to just slowly adding some new, without endangering the old, with no true horrible outcome.
Daring as that could be for most; there is a time and place for that, out in the open where maybe Una doesn't change, doesn't mean things around her don't. Outside that bubble's of her, everything is always changing, and if she really took noticed of that. The warnings would have been caught sooner. Una could have saw beforehand or overheard from others, of how the sewer maintenance was taking too long to fix something.
Lazy enough they were, on this human error to not even put up warnings around their work place, while going on break. Leading not long later, for the young Una, who decided walking backwards home for the day was great to mess around with, even with wanting to get home quickly for dinner. Una still wanted to have her day to herself, before going home.
Another human error on her: for not hearing the cries for her to stop, as with a sudden motion she was falling. So quickly to open her eyes, to noticed everything rise over her, as she fell straight down, arms trying to reach out for something to grab. Yet, everything was still slick from yesterday's rain; something to save her was pointless.
Even the maintenance ladder was moved; she had nothing to save her, as she fell feet first into darkness.