Interesting set of replies I must concur that it was tube caught between the bead and rim.

I remember my father teaching me to repair a puncture. Our little lane had a hawthorn tree halfway along it, so punctures were a regular occurrence.

When reassembling the wheel, the following steps would be used:

Fit the tyre over the rim with one bead in the rim groove.
Install the inner tube into the tyre, starting with the value being inserted into the rim.
Pump in just enough air to make give the inner tube shape, but still be very soft (this was the critical bit to stop it getting caught between bead and rim).
Then work round the tyre bending the tyre over to pop the bead into the rim. (no tyre levers needed to refit the tyre, only to remove it)
Now work round the tyre bending the tyre over to check the inner tube is not visible between the bead and rim first one side, then the other.
Pump in a bit more air and then bounce the wheel working round the entire circumference to settle the tube.
Fully inflate.

Following this process and I have (to date) never had an inner tube failure or puncture repair failure.