Does this really make you worry that they would tamper with information that mattered? Seems a bit far-fetched to me, doing that would be massively scandalous, and probably illegal.
If they are then they are tampering with information that mattters, SELL is a valid stock descriptor (SELL.ME, selestra corp: http://finance.yahoo.com/q?s=SELL.ME).
Financial data sites are not a good place to be pranking your users.
That alone makes me seriously doubt that this is an easter egg, if it is google will have easter egg all over its oos.
