Separating Out in the Tube
Solution: Live with it and mix the pigment/binder back together with a palette knife. Or contact the art store you bought it from for a replacement and, failing that, the manufacturer.
Paint Drying in Tube
Solution: Ensure you put the cap back on the tube straight and tightened all the way. Do it timeously; don't leave a tube lying around open, especially in a hot environment. With plastic tubes, try to avoid getting air into the tube.
Not Covering What's Underneath
Solution: Swap to opaque pigments, or mix in a bit of titanium white which is extremely opaque.
Color Shift from Wet to DryDepending on the brand of acrylics, and more so with cheaper paints than artist's quality, you may encounter a color shift from when the paint is wet to when it's dry. It may get darker as it dries. This can make mixing a color again to match tricky, and make a painting turn out much darker than you'd intended.
Solution: Upgrade your paints to a better quality. Learn through experience how much a particular brand darkens, and learn how to compensate when color mixing.
Drying Too Fast
Solution: Check if there's a draft across your canvas, whether from a window, fan, or air-conditioner, as this will speed up the drying time of acrylic paint. Use a fine-mist spray (Buy Direct) with water over your palette and canvas regularly, or mix in some retarder medium (Buy Direct).
Not Drying At AllIf you've mixed retarder with your acrylic paint and it's now not drying at all, you probably added too much. Check the label of the retarder to see what the recommended proportions are.
Solution: Try to remove as much of the paint that's not drying as possible.
Dried Paint LiftsIf you find that paint you thought was dried lifts off the canvas when you paint over it, chances are it didn't have sufficient binder in it and too much water.
Solution: Thin your paint with glazing medium (Buy Direct) not only water. Paint over the area gently with a layer of glazing medium to try to seal it without disturbing it too much.
Foaming PaintAcrylic paint has additives in it to reduce foaming and froth, but sometimes you can end up with a frothy mix. I've mostly encountered it when mixing paint with mediums vigorously. (See also: Taming the Foam from Golden)
Solution: Wipe it off with a cloth, clean your brush and start again. Or ignore it and if the paint dries with any bubbles or splotches, let it be part of the painting.
Paint Isn't Glossy
Solution: Mix in gloss medium (Buy Direct) or, when the painting's finished, apply a few coats of gloss varnish.