A little extra B12 is easily flushed out, but when it's a lot, meaning several milligrams per day for many days, it can quickly and suddenly cause very elevated blood pressure and heart rate, e.g. 160 for both SBP and HR. This effect is easy to undo using a good beta blocker, e.g. atenolol, but if one doesn't, then an emergency hospital visit is required. The point is that despite being water soluble, this adverse effect does happen.

Even at just one milligram per day, it can significantly harm sleep.

It has a high circulating half life of six days, which implies that it can really build up if repeatedly megadosed in this period.