Weird SignalTap behaviour

The previous discussion was about unconstraint and possibly too fast Signaltap clock rather than metastability. I explicitly mentioned that metastability won't cause fake signal transitions for stable signals.

Not knowing any details about your design, it's impossible to relate it to the present discussion.

I rarely see unexpected behavior of Signaltap. Few cases I remember are failure of trigger or storage qualifier logic with signals asynchronous to the Signaltap clock. I don't remember a case where a known stable signal showed fake transitions, except for situations where the acquired signal couldn't be correctly transferred due to JTAG signal quality issues. In the latter case, I got different recording each time the signal was retransmitted by pressing "read data".