As FFR is a game run in Flash (running at 30 FPS usually), the spacing between arrows in streams are not the same all the time. In a long string of notes (also known as a stream), the notes' spacing are separated by a certain amount of frames. The number of frames usually vary and it depends on the BPM of the stream. Since the frame gaps aren't always the same throughout (aside from several BPMs), it makes it almost impossible to hit full amazings in a stream. However, the percentage of n-frame gaps are not random and can usually be found out through some simple mathematics.
The probability of a stream having a n-frame gap varies on the BPM (or beats per minute), as mentioned above. To find the probability of an n-frame gap in a 16th stream: $ 450 / BPM = Average frame gap $
This doesn't only apply to streams, but to bursts and jacks as well. The formula is still the same, however.
Although we do know the probability of a burst/stream/jack to have an n-frame gap, it doesn't change the fact that we don't know for sure whether this burst/stream/jack will have an n-frame gap. Because of this, files end up having what are usually called in the community, framers. Framers are basically frame disreprencies that create very noticeable artificial difficulty. There are mutliple files with these "framers", but they are usually present in files that are extremely bursty or have ridiculously fast rolls (or walls), so they're only present at files at a very high difficulty, usually 70 or higher. A couple of files with "framers" are Mourning the Lost and Integraation. Most files at the higher-end have their framers fixed, such as grind2 and Kirlian Shores, but there are still a lot of files out there which still have "framers". In official tournaments, these framers are fixed before it is released in the tournament to ensure that the difficulty isn't artificially inflated.
0-Framers are basically notes that are supposed to be separated but they appear to be on the same frame, which is why they're called "zero-framers". 0-Framers usually appear when 2 notes are extremely close to each other (equivalent to 450+ BPM), which doesn't cause much issue to most players. However, the community uses the term "0-framers" for minijacks. Due to the fact that a minijack has a 0-framer, it will look as if the minijack is just a single note, but you will get 1 miss if you only hit the note once. Because 0-frame minijacks are so fast, it's almost impossible to hit perfectly (without resorting to overlay or double setup) and prior to the av-miss glitch, it was unhittable, regardless how fast you hit the second note. There were a couple of charts which had 0-frame minijacks, but the only one that can be played on FFR is Party 4u "holy nite mix".