The first thing you need is to know what's going on. Monica thinks her maid is stealing. So far,the maid has only stolen her pants as far Monica thinks. So now Monica is trying see if the maid has stolen her pants- And she'll know if she has because theirs a stain on the crotch.
Now the slip-up takes place in the kitchen,after she confronts Chandler that she thinks she's stealing.So now in the kitchen she's spying on the maid to see the stain. Monica says, "I like your pants," and the maid responds, (or something like it) "Oh thanks.I like your shirt." Monica says, "Thanks," then mutters, "You're not gettin' it." After Monica said 'thanks',she wrapped her arms around herself,and pinches her shirt,revealing a white bra.
Then,later on the show, Monica is with Chandler and notices she's wearing a pink bra-which Monica thinks she stole. Monica leaves Chandler alone to see her bra,and Monica waits outside in the hall. Monica realizes that she herself is wearing the pink shirt,which 'she as been looking for for weeks'. So it has to be a slip-up. If it wasn't,it'd be like she was wearing the white bra,put on the pink during a time lapse,and then saw the maid wearing the pink bra,and for some reason Monica wouldn't realize she JUST put on the pink bra,too. It doesn't add up.
SLIP-UP - and they could've caught it!