To reproduce: Focus your crosshair on a block, then type in any of the following partial commands and attempt to tab complete the following coordinate arguments:
/replaceitem block
/spreadplayers
/stats block
/worldborder center
Since /tp has two ways to be formatted, it might not be desirable to add tab-completing for coordinates in this case, but for completeness' sake:
/tp @p
will not allow tab-completing coordinates, likely because the destination player argument could follow just as well as coordinates.
Confirmed for
1.8