pretty easy to get bad (negative #s) in dbtop like:

NS total Reads Writes Queries GetMores Inserts Updates Removes
GLOBAL 8264 60.3% 0 0.0% 8264 60.3% 82 11.6% 0 0.0% 8182 48.6% 0 0.0% 0 0.0%
test 82 11.6% 0 0.0% 82 11.6% 82 11.6% 0 0.0% 0 0.0% 0 0.0% 0 0.0%
test.foo -18 -0.1% 0 0.0% -18 -0.1% 0 0.0% 0 0.0% -18 -0.1% 0 0.0% 0 0.0%

here is a script which will do it. press reload several times while running until manifests

S:\cygwin\home\dm\mongo>cat test.js
t = db.foo;

while( 1 ) {
for( var i = 0; i < 100; i++ )
t.insert(

);
t.drop();
}