I got this flat on brand new tube , right after riding less than one mile.
There are no spokes sticking out, I checked it twice. There is no sharp object either.
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Its right by the valve hole, so grab a ruler and measure how far the hole is from the edge of the valve.
Do the same measurement in your rim and it will prove or eliminate the first spoke hole as a cause.
Clearly the green slime sealant didn't do its job either.
There's also a chance it was a manufacturing defect, with a weakness in the butyl rubber that held pressure only long enough to get down the road before letting go.
This may not be patchable either - I'd certainly try, but you will need to clean the sealant off the area (not just a wipe)
And the proximity of the valve base to the hole, as well as that chunky ridge will also make it hard to patch.
If you're positive the rim strip / spokes / spoke nipples is not the cause, then fit a new tube from a different manufacturer and see what happens.