An Australian study came to the conclusion that moderate amounts of coffee can make us become easily influenced and persuaded.

A study conducted included people who were asked what they thought about voluntary euthanasia before and after reading persuasive arguments against their initial beliefs. Just before reading the arguments one group was given orange juices that contained caffeine in amount equivalent to two cups of coffee and another one was given placebo.
Those people who had caffeine juice had a better memory of the arguments, showed increased agreement with the arguments and had argument-related thinking.

I am sure you would think twice before having a cup of coffee before an important meeting, argument with your partner, or just before making a big decision about buying expensive stuff.