My NDNs are on holiday for three weeks and before they went away they asked if I would be able to pop to their allotment, which is near where I work, twice a day. They're lovely people and feed our cat/keep an eye on the house when we're away so I was happy to say yes.
What they need doing isn't onerous; they're not too worried about the stuff that's planted out but they have a greenhouse full of tomato plants so they need a couple of greenhouse windows opened each morning and the 'mat' thing they stand on watered, then the windows closed again in the evening and the mat watered again.
The morning visit is a little bit of a faff as the allotments are about 3/4 mile past my workplace and I use public transport to get to work so I have to leave earlier, get off a couple of stops further on and then walk or get the bus (depending on weather / time) back to the office.
I have a friend who lives a few streets away from where the allotments are. Last time I saw her for coffee, a few days before NDNs went away, I mentioned in a jokey way that I'd be hanging round her neck of the woods and when she asked why, I explained about calling in to water NDN's tomatoes.
On Monday night she sent me a text saying "Just thought, I'm on holiday this week so could do the morning allotment check for you if you wanted". If NDN had left me in charge of something v valuable or v complex I would have thanked her and declined, but as it is literally opening two windows, turning on a tap and turning it off again, I replied with grateful thanks, the (fairly basic instructions) and very clear directions to NDN's allotment - not hard to miss, they have a couple of v distinctive features. And then more thanks.
When I called in on Tuesday evening, the greenhouse windows were shut and the mat looked fairly dried out. Same again yesterday. I even looked around in case there was another allotment with similar 'features' and a greenhouse that she might be getting confused with, but there doesn't seem to be. She doesn't seem to have had any family catastrophes that may have prevented her, as she has been active on FB with her usual chatty inconsequential stuff.
It's fine, it's no big deal as I was prepared to do it for the whole three weeks anyway but it is slightly irritating. Why would you bother to offer to help someone out and then just not do it? Just don't offer in the first place!
(And yes, I've been too wimpy to ask her why she didn't do it because I don't feel I can remind her about a favour she volunteered 
)