Among some circles in the drug culture (especially the club-drug culture, but not limited to it), referring to something as a "little white pills" has become jargon for pills whose contents are either unknown or indeterminate, usually signifying that no one in the area has tried/tested one, or else they have been accidentally mixed with similar-looking tablets. Use of this phrase as vernacular usually signifies that the speaker is not certain of the pill's content.
Examples of usage:
"I accidentally dumped the rolls into the bag with the ephedra, now they're all just little white pills."
"We went to (insert name here)'s place to roll, but all they had were little white pills, save your money."
"Before we leave, let's separate the little white pills from the rest of the stash, until the guy with the marquis reagent gets here."
As with most other forms of jargon or vernacular, existence or usage varies with geographic location (a good example is "candyflipping (New England) == Trolling (South/Midwest)").
As of the time of this noding, I am unaware of any synonyms for this saying, although I've heard the word "HeisenPill" used a couple of times for pills with "uncertain" contents (Get it? Heisenburg? Uncertainty? I thought it was funny), or situations in which devoting the necessary concentration to examining the effect a specific pill is having on your thought process would alter said effect.