Imaginary things that junkies do, Number One.
When prices of gabo get particularly high, your average junkie cannot afford to waste even a speck. He or she is probably scoring less gear than they need to feel good, due to the temporary price rise, which can be caused by a series of raids, or perhaps a manufactured shortage, (where a nation's government says that it has stopped ("dealt a death blow", the media like to say) the trade in narcotics). Whilst this is a ludicrous statement, it cannot be challenged as any government will say that they cannot reveal details of the recent busts as there are ongoing operations that could place their operatives in jeopardy if word got out.
In other words, piffle.
Anyways, if gear gets very expensive, the average user is left scoring less than they need. Here's a trick some use to ensure they waste not a drop.
When preparing a shot, the pennypinching junkie will leave 5 units of air at the top of the sharp. The user is able to push every micro milligram of the gear they have just bought into their arm or other point of entry. The bubble is a great visual indicator, you can see what's going on very easily.
Be careful not to push in the air bubble - street mythology is that it will stop your heart. Medical texts I've read say that you need at least 5mLs of air (500 units, or 5 full 1mL sharps), but don't take the risk. Just watch the bubble, and when you see the last of the liquid dregs pass by, that's time to pull out.
You won't waste a drop!