One of the most common things you'll see in ufology is ahistorical "reasoning". Some of it has popped up in the comments to the MJ-12 / Landry post a few days ago. Folks who make this error usually mean well, but what they're doing is taking historical events out of their proper context and trying to fit them into their own view of how the world works today.
An example would be something like:
"Well, George Bush did X, which means that a president fifty years before must have or probably also did X." Or words / meaning to that effect.
Or this (one of ufology's favourites):
"Our intel and military services act like X today, which means that they must have acted like X in 1947."
And so forth.
History students are taught from day one to avoid this kind of "reasoning", but even there it rears its head more than you might think (I marked enough undergrad history papers to know whereof I speak - indeed, I've been guilty of it in the past myself).
Ahistorical analysis can be difficult to detect sometimes, particularly when the time periods are relatively close to each other, as is usually the case when the conversation is about UFOs (the difference between 1947 and today, which spans the modern UFO era, is a drop in the temporal bucket).
But try it this way:
"Our intel and military services act like X today, which means that they must have acted like X in the Revolutionary War."
That doesn't make much sense, does it?
This does not mean that there are not common threads that run through history. But these are patterns / conclusions that are drawn from common facts, not the other way around.
When examining historical events, we always have to remember to view them in their proper context, and not view them through the prism of our own time and experiences. In other words, we should stick to the facts as they were, not as we would like them to be, or think they were, and draw our conclusions from that. We can still be wrong (facts often support more than one reasonable interpretation), but at least we'll have gone about the process in the proper fashion, which always decreases the chances for error.