Can I get charged with shoplifting before leaving the store?
If you are in the process of taking something without paying for it--for example, stuffing it under your shirt or in your backpack or handbag, or removing the theft-control label from it--you can be charged with shoplifting even if you never successfully left the store.
You can be arrested for a crime even if the crime was never successfully completed. That, for example, is how the authorities arrest would-be terrorists before they ever go on a shooting spree or set off their bomb. All that's necessary is that there be sufficient evidence that you were in the process of committing a crime. So long as there is, you can be arrested, charged, and even convicted.
In the high-profile terrorism case, that is generally evidence of acquiring the materials or supplies for a bomb, or guns and ammunition, coupled with writings, communications, statements, comments, etc. showing that the intent behind doing so was terroristic, not innocent. What about in a shoplifting case, though?
Clearly, simply holding the store's goods is not enough. That's what shoppers do, before they check out and buy the items. But normally, when shoppers hold an item prior to buying it, they don't conceal it--they hold it in plain sight or in a shopping basket or car. They don't take any steps to remove any theft control tags or circumvent other security measures. And they also don't have a friend or accomplice distract the store clerk or security as they (the one holding the item[s]) move towards the door (rather than going to the register to purchase the item).
Therefore, if you do something that shows you are attempting to or are in the process of shoplifting, such as stuffing an item under your clothing or placing it in your backpack, briefcase, or handbag; or removing any security devices, or wrapping the item in foil to try to defeat the sensors; or begin walking out while your friend or associate distracts store staff, such actions can demonstrate that you are in the process of shoplifting and allow you to be arrested and charged even if you never completed the theft and left the store with the item.
The law--that is, the authorities and the courts--are pragmatic, not dogmatic. If you are doing what any reasonable person would conclude was shoplifting, you can be charged, even if you never succeeded in removing the item from the store. The law, quite simply, does not require that a property crime (for example, theft) be successfully carried out in order to hold you liable—that is, charge you—for it. (Crimes against a person are different: if you tried to murder someone but failed, you would not be guilty of murder, though you would be guilty of attempted murder.) The "technicality" that the item is still in the store will not save you.