Pitch velocity is the only stat I can think of that is subject to this, though. Every piece of data in baseball is pretty cut-and-dried. You can't fake the number of at-bats someone has, or the number of hits or RBI. You can't fudge batting average or ERA, because they're based on physical acts that are plain for all to see. A GM or a scout or an agent can't claim a player has a higher average than he actually does. You can't mess with the data on things that happen in a game, because they're recorded.
The only two things I can think of, besides pitch velocity (which is meaningless in the grand scheme of things) that are subject to human interpretation are balls and strikes, which don't factor enough into any statistic to be noteworthy, seeing as they have been subject to human error for over a century with little global impact, and scoring decisions on errors vs. hits. That is also negligible, I'd say, since the percentage of scoring decisions that could go either way is low (most errors are quite obviously errors, even to those of us who are not players or personnel). Small changes or ripples tend to factor out over time, that is a principle of many branches of science and history and it is, I think, an excellent principle when applied to baseball.
Statistics are real, at least in baseball. Whereas statistics in most fields, like politics or other demographic minutiae, are subject to errors and malicious interpretation, sports statistics are, by and large, pure and unadulterated. If a player has 1000 at-bats and gets 500 hits, he (in addition to being the best batter ever) is batting .500. If a pitcher gives up 3 runs in 9 innings, he has an ERA of 3.00. It's all math, and math is incorruptible.