Ever come to use a new paint or one that hasn’t been touched in a while to find that the pigment has all settled to the bottom of the bottle. Most of the time this will result in either 15 minutes of vigorous shaking followed by wrist ache or giving up and using another colour instead.
To avoid this problem, make sure to check your paints regularly. If any appear to be settling simply rotate them to a different position, such as laying them on their side (I store my paints standing up and have a little box I use to stand them upside down). Once the pigment has shifted give them a quick shake before putting them back in their rack or whatever you keep them in.