Is moldy wood a problem?

A few months back I built this workstation http://plansnow.com/dn3082.html
When I built It I knew the plywood piece that was guled into the 2x4 didn't seat very well. It didn't matter It was just a temporary table. Well the 2x4 popped loose at one end so I pried it completely apart for re-gluing.
In the dado of the 2x4 under the dried glue was obvious round spots of dark mold. None of them very big, the largest may have been 3/16. This was standard construction lumber so it was probably a little moister than kiln-dried hardwood but not dripping like PTL. The piece was stored in a dry garage and was never exposed to rain. The mold was only visible inside the dado and not on any exposed surface. Did the moisture from the glue cause the mold? Anyone else ever discover hidden mold in a project?
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Mildew. You wet the joint, then sheltered it. Doesn't take long at 16% or so moisture content.
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