I occasionally get canker sores in my mouth. Sometimes they appear without any apparent reason, but they most definitely show up if I get any sort of minor (or major) irritation in my mouth (like a cut or abrasion). Canker sores usually take about 7-10 (or 14) days to go away.
I have prayed against the past several canker sores I've gotten, and the duration of the outbreak has been shortened by several days as a result. On the last one, I prayed that the virus causing the outbreaks would come out from hiding, be exposed to God's glory, and become deactivated in Jesus' name.
I noticed that I got a little cut on the inside of my lip about 4 days ago. Interestingly, that little cut did not turn into a canker sore, and it has in fact completely healed. I am not sure what to make of this. I cannot remember a time when I got a cut inside my mouth that did not turn into a canker sore within a few days. I want to be wary not to jump to conclusions based upon one case, just in case this happens to be a lucky coincidence. Nevertheless, I praise the Lord even for little things like prevented-canker-sores!