Oleg Rekutin, it's tough to characterize exactly how this bug might have exhibited.
The bug was triggered if a database page was not in cache at the beginning of a checkpoint, then it was read into cache and dirtied (a document was inserted, updated or deleted) during the checkpoint. The checkpoint would (correctly) skip over the dirty page because the changes were too recent to appear, but the tree containing the dirty page could end up being marked clean. Then if there were no more updates before mongod was shut down, the dirty page would never get written.
Once the bug has occurred (i.e., mongod has been shut down), there is nothing in the data files that could be used to detect or recover from the lost update. As noted here, this was fixed in MongoDB 3.0.1.