I grew up in, and am still a part of a fundamental Christian fellowship which allows laymen to pray. And I do mean laymen; women are not allowed to take part in the public worship service, but that is another story for another day.
Being allowed, and even expected to pray publicly in worship, especially in front of a large audience, can be daunting, especially to this newly baptized 12 year old. My Sunday School classmates knew this and we played mind games with each other. "Just be sure you don't thank God for the sick," we'd say to each other. The purpose was to psych each other out with a phrase we didn't want to say, but was close enough to all the other prayerful vain repetitions, that it was easy to let slip out.
It was ok to thank God for all the other blessings we believed He had given to us, but far be it from us to thank Him for those with maladies. I'm not so sure why this was such a big deal back then. After all, Christians should be willing to forgive and overlook this type of misspeak.
Years later my congregation did start up a training class for young people so that this type of thing would be less likely to happen.