I picked up some presta tubes for my partner’s bike and mistakenly came away with long valve tubes (the picture on the boxes showed a short threaded valve, it’s only when you read the small text on the rear that it mentions long valve).

Apart from looking a tad silly will there be any problems running these on a mountain bike (as 26″ tubes they’re clearly meant for mountain bikes); the valves aren’t threaded and so have no lockring..

No problem at all. I try to by these anyway as the extra length was needed for 317/717 type rims.