If it was there when the spreader and CPU were first put together, it'll probably stay there, maybe crushed. If the spreader and CPU weren't correctly assembled, with, say, one corner not having adequate pressure, maybe. If you have taken the spreader and CPU apart, maybe.
Even if you have used too much thermal compound, the excess should have just squeezed out at assembly time.
Otherwise, I would say no, as the pressure will be too much for most things to get in. The only exception I can think of is you've soaked one side of the heat spreader in, e.g., some kind of acid that dissolves the heat spreader material - that would leave a gap that could allow something in, although you'd probably be worrying about bigger problems (like where did the acid come from? what other damage did it do?).
I've just cleaned up the Celeron with Articlean. I usually have the top of the heat spreader swimming in the stuff prior to soaking the majority of it up and then wiping away the TIM.
I've done this a million times before and never had any issues but today whilst wiping away a little TIM that got on the lip the coffee filter (yes coffee filter!!!) came into contact with the Articlean and managed to get onto the board.
Some of it was around the little notch so I'm a little worried that some of it may have got under the heat spreader.
I've cleaned it all up. Having a nightmare today. One of the stock HSF locators is goosed as well so just chucking on a Sonic Tower as we speak. My ASF 5's a little runny as well but should get away with it.