2018 UPDATE: There are not many smartphones left that have user accessible batteries. This applies to older phones.
If you are an Android smartphone user and you notice your phone acting strange, it could be a faulty battery. When I had my Samsung Galaxy S2 I had frequent freezes, reboots, and crashes. I had reset the phone, but the weirdness continued. I finally took it in to a local Sprint service center. After leaving the phone with them for a couple of hours I returned to find out that my phone was just fine, but the battery was bad!
You know how bad canned goods swell up on top? Batteries do the same thing. When I looked at my battery it was clearly swollen instead of being flat on both sides. It may be a bit hard to tell on the photo, but the middle of this battery was clearly puffed up. This can happen to batteries over time. At that point I think I had owned my phone for over a year. This can also happen when using cheap chargers that may not regulate the power correctly and overcharge the battery. I ordered a new battery for around $20, replaced the bad one, and my problems were solved!
A Facebook friend recently experienced the same thing with her Galaxy S4.
So, before you take drastic steps to reset your phone to default and/or take it in for service pop out the battery and see if it looks ok.