Obsession is something that occurs in all walks of like, but doubly so within the occult world, where there are many things to fixate upon, and many ways for the magician to lose control and end up being controlled by outside forces. Mathers and Westcott frequently issued warnings about this.
A student might, for example, feel a special affinity with a number. Let us say 10, since it is the number of Sephiroth. Then they might start to obsess about it, seeing it everywhere, and thinking there is some greater significance than that which they are affording to it from their own behaviour. They suddenly note that they stopped reading a book on page 10 (despite ignoring that they stopped on page 6 the night before), keep noticing the number 10 bus (despite ignoring all the hundreds of other numbered buses they encounter perhaps more frequently), and start to let the number rule their life. Then the occult meaning is irrelevant, as the true value of the relevant teachings of that number have been supplanted by a kind of paranoia.
Numbers are particular foci for people within the occult community, thanks to the importance of gematria (a system that is much abused, leading many occultists to force meaning where there is none). Someone may glance at the clock at 11:11 and think it significant, or they might stop on page 93 of a book and see it as a sign. In numerous cases, the more they appoint significance to these encounters, the more these encounters appear to happen (or, rather, the more they notice them, at the expense of noticing all the other numbers that are deemed less significant).
This is particularly common with numbers that have strong esoteric associations, like the number 93 of Thelema. In answer to this, some have attempted to give the Golden Dawn a special number, employing 120 due to its Rosicrucian associations (Post 120 annos patebo), but I disagree strongly with the search for a single number of importance, when instead we should seek to learn from all of the numbers, and never be ruled by one of them.
This is why even meditation upon and skrying of symbols requires much care and many safety techniques. It might seem harmless at first to think of a special affinity with, for example, the symbol of Venus, but if it stops becoming a means to an end (the acquisition of greater knowledge and understanding), and instead becomes the end itself, the focus and fixture of the mind, it becomes a detriment more than a stepping stone to greater things.
Indeed, even a positive and protective symbol like the pentagram can be obsessed over, leading Moina Mathers to write:
"Once the symbol has been clearly traced, it is better not to think of it too much; for the Neophyte ought not to permit himself to become obsessed by any Symbol."