Hi there!
Usually I write citations immediately, and I guess it is due to my whole process of reading and writing since I usually have all the citations at hand.
I had a huge problem last year - I was writing a text and had left it unfinished for a month or so. I had inserted several paragraphs from other authors, wanting to rewrite them and cite them properly. However, my hard disk died in the meantime, and luckily I had most of the stuff (my text and the literature) on cloud, but few of those articles were not on cloud. Anyway, when I got back to finishing my paper those non-cited but inserted paragraphs were from the articles that were lost with the hard drive, and I have missed them entirely when I checked my paper. I've sent it for publication, and was duly noted by the editors that there was suspicion of plagiarism, but luckily I explained the situation, corrected my mistakes and everything was fine at the end.
After that experience I am very much inclined to finish citations when I create them. :)