- Constant changes, which may involve a number of factors coinciding including data refreshes, different datacenters using different datasets and/or algos, filters being tweaked, knobs being turned up or down. These changes are seen several times a day, every day, for months now. Forget about personalizaton making it difficult to track rankings. The constant changes alone makes it difficult to track rankings.
- Pages suddenly becoming uncached, that have previously been cached for a long period of time; most recently, with pages that are a root of a subdirectory, such as mysection/, products/, etc.
- Different pages other than the "best match" being returned in the SERPs, usually with a much lower ranking. Looks like Google gets confused about which page should be returned. Normally, the best match page is returned at some level of expected ranking (usually high), but sometimes the best match page disappears from the SERPs, and some other page reappears, with a much lower ranking.
- Some rankings are on a cycle. People see rankings disappear on a Monday for example, only to return on a Wednesday or Thursday, disappear again the next Monday, reappear the next Wednesday, etc. on a constant basis. The days of the week here are examples only, and differ with each person. But for each person seeing this, the cycle is very predictable.
What have you been noticing? Anything to add to this list?