My guess would be it is just a damaged one Nariz and the next one will be back to normal

You may have a small plant growing in the same pot as the big one, and its a slightly different variety??

No, Pam - it's the same plant. I just checked the surrounding plants and found that I have an ivy-leaved pelargonium nearby which is a darker shade of red - maybe cross-pollination? Or, as Drc says, damaged.

Yea, that could be it then :)

the cross pollination would only affect the out come of the flowers from any seed made. The amount of water and temperature can influnce the colour and form of the flower. others will be more 'normal'.

I notice that some of the leaves in the vicinity of the weird flower are different than the others--less conspicuous zones--so I would expect another plant in the pot. Sometimes you have to look very closely at the base, moving aside the lower leaves.

Thanks, but there is only one plant in the pot - I have already carefully checked for the obvious. We have had blistering temperatures followed by a cooler week, then high temps again - perhaps Seaburngirl is correct?