Not only is it not unlikely, it's extremely likely and has SHOULD have happened multiple times.

It's not 0.04% of it happening ever in the lifetime of the universe.  It's 0.04% chance *on any block* that the pool solves.  Slush has probably around 25,000 blocks under its belt in the history of the pool.  Meaning it could have happened 10 times on Slush and it would be *exactly* within expectations.  And with such an unlikely event and a sample size of only ~10x the expected rate of occurence, even if the pool has had 12 rounds at that 99.96%+ CDF, it would not be a statistically significant deviation from expectation to cause alarm.


(Note:  25k is probably a conservative estimate used for illustration purposes, estimated by using how many blocks BTC Guild solved in its lifetime, which is shorter than Slush's pool lifetime, but BTC Guild was larger than slush for the majority of its existence, so they're probably similar in terms of total blocks solved).



EDIT:  Actual number:  24,580 based on the Slush website block ID.  I know there's been a few IDs that were skipped due to a block being put into the database multiple times, so lets say 24500.  Pretty much changes nothing stated above.