There's a video clip going around like crazy the last few days which purports to be "proof" of a time traveller spotted in the 1920s. The pros and cons of the "argument" are easily enough weighed, and doing so offers little disincentive to those who are the kind of people prone to believing such things. I won't repeat all that discussion here.
What I find intriguing is one point which I can't really elucidate in the short space that typically is all that's available in these discussions, and certainly not without coming off as combative, so perhaps this is a better venue.
The entirety of the argument for this person being a time traveller is the fact that her motions look familiar to a modern-day person familiar with cell phones. She appears to be talking while holding something up to her ear as she walks down the street, and that's something we all see every day, but which just 15 years ago would have seemed almost inexplicable. This is the key point: there is nothing else about this film clip that is purported to suggest anything other than the specific resemblance to today's cell phones. (Or at least their behavior. We never actually see what may be in her hand, we just see that her hand is near her ear and she's speaking -- which is far more easily explained using the hearing aids of the period, but nevermind that.)
Cell phone technology is probably the fastest-changing technology that an average person deals with today. Today's cell phones don't even closely resemble those of five years ago. Products are already on the market right now to disguise Bluetooth headsets as earrings or other innocuous items, and work is being done on various forms of implants. People are doing more texting than talking already, and that's even with things like Twitter in their relative infancy. It is a virtual certainty that, five years from now, the cell phone experience that this clip purports to show us will already be rare, and twenty years from now, the odds that anyone will be walking down the street holding something to their ear and talking are nearly nil. Yet we are certainly more than twenty years out from time travel.
When someone points things like this out (or the question of exactly what cell network this alleged time traveller is using back in the 1920s, and how many bars she's getting) people can come up with the most fanciful explanations. Perhaps they've invented some way to relay cell phone signals through time... and yet, it hasn't occurred to them to just use something other than cell phones? Perhaps somehow they've invented time travel yet coincidentally forgotten how Bluetooth works. Perhaps she's actually talking on an intertemporal walkie-talkie that just happens to look like a modern cell phone.
What all these explanations, if you can call them that, miss, is that the only reason we were even considering this clip as evidence of anything was the resemblance to today's cell phones. Once you have to propose a complex situation in which it's not even really a cell phone, or some unlikely scenario by which the time travellers of Star-Date Three Thousand And Something happen to use something that looks like a Nokia phone from 2007, you have already given up the only thing that made this clip noticeable in the first place.
I can certainly think of a fare more plausible explanation than any of those I've heard, and that is, if you're time travelling, perhaps you disguise your mission equipment as period-appropriate props, but this particular time traveller got stuck with a prop prepared for a 2007 voyage while on a 1920s voyage. After all, if you saw a clip of someone in 1925 holding a 1954 Regency TR-1 transistor radio, that's the story you'd come up with, not the idea that time travellers from 1954 were visiting 1925 and listening to their Amos 'n' Andy broadcasts via transtemporal retransmitters. (And yet, a 1954-model transistor radio is hardly any more unlikely for a time traveller to be using as her native equipment than is a 2007 Nokia cell phone.)
This is a thousand times more sensible than the idea that time travellers can erect temporary time-relay cell towers in hidden locations so that they can continue to use their vintage iPhones while visiting ancient times, and yet, it's still so incredibly unlikely compared to such mundane explanations as "she was using a hearing aid" that I can't understand how people can take this seriously.
Using this clip as a brilliant hook in setting up a roleplaying game or writing a story: good. Using it for some amusing exploration of imagination: good. Actually believing it's evidence of anything at all: precisely the kind of irresponsible insanity that we cannot afford in a time when potentially intelligent people are revelling in their ignorance and being shoveled more of it every day, and critical thinking is almost a forgotten art. Those of us who do the former should be far more careful than we have about making sure our imaginings aren't mistaken by the Unthinking Masses as evidence, because they won't know any better.