This won't be news to anyone who has been using an adequately-rated power supply for their Pi, but, a typical cheaply-made cell phone charger allegedly rated at a nominal 5 volts at 700 ma may not be enough to ensure stable operation of the Pi. This is especially true if the micro-USB cable is not constructed with conductors of adequate gauge (wire thickness) and/or the cable connectors don't make good enough contact under full load. We had been experiencing at least once-daily mysterious crashes on a Pi in the UK I've been accessing remotely from California.
After the owner upgraded to a 2.1 amp supply with a more substantial cable, we've been running for several days with no more crashes. What is particularly curious is that the Pi would crash even with nothing else running besides an xterm window on LXDE on the Debian 13 April release, with nothing executing in the xterm window when the crashes occurred. In theory, the Pi should have been drawing well under the 700 ma maximum stated in the Pi hardware power requirements spec.
I had previously turned off the screen-saver, thinking that some of the more graphically-intense savers might be pushing the Pi to the limit of current capacity of the cell phone power supply, but, that was not the case. The GPU wasn't even being used during any of this period, which is even more surprising. The owner hasn't had the chance to measure voltage and current output of the original supply under various Pi operating loads, but, it's pretty clear now that it was the culprit.
The moral of the story is that, even if you have a power supply that is allegedly rated at an adequate voltage and current for the Pi's requirements, it may not actually be delivering the required power in terms of voltage and/or current. You cannot have too much current capacity as long as the power supply is providing a regulated 5 volts (+/- 0.25 volts, i.e., 4.75 to 5.25 volts under load). Any current in excess of the Pi's requirements can be routed to an active USB hub, so, upward of several amps of current capacity would be just fine.
If I ever receive a Pi board, I will be taking it to all of the cell service providers in my area and trying to power it with as many different micro-USB equipped device chargers as are available, running the Pi up to full operating load, including the GPU. I'll publish a report by both phone and charger manufacturer, where it's specified on the charger, along with the current ratings, which should be printed on the chargers.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!