Some Spanish government bond yields dipped below U.S. Treasurys, adding another twist to a year of surprises for the world's bond markets.

Yields on the 10-year Spanish bond fell to 2.579% Monday, according to Tradeweb, lower than the 2.615% yield on the 10-year Treasury note for the first time since April 2010, before the euro zone tumbled into its debt crisis.

While...