One of the things that I noticed, starting as an undergraduate and moving into graduate school, is that scientists have a peculiar sort of conservative language. They'll say things like, "it's possible that such and such", or "it's likely that such and such" but rarely (if ever) say "such and such is true". In a previous postI said that a proposed solution to the faster than light neutrinos was likely to be true, but left open the possibility that the error will probably be found elsewhere. As solid as relativity is, scientists will still never say that relativity is "true", only that is it very well established, confirmed, supported, etc... In any other context, something with as much support as, say, the theory of relativity or the theory of evolution or the germ theory of disease would simple be called "fact". In science, calling something like that a fact is both wrong (it can still be modified!) and dangerous (it stops all inquiry!).
Paraphrasing the words of Sam Harris, pretending to know things that you do not know, or claiming certainty for things for which there is not evidence to support it, is a huge liability in science...and should be a liability in other contexts as well (think religion, politics, etc...)