Causes of inconsistencies can be elusive

First of all, whatever you do, don't just dismiss the screen, investigate and solve at once! Forcing Hysys to continue calculating in some instances changes the object Hysys points to as the cause. It then becomes a lot more difficult to solve the issue.

The obvious suspects:
Pressure, temperatures or flows in streams.
Vapour fraction: you specified vapour fraction 1 or zero becasue you knew the fluid was all vapour or all liquid. durng the solution pass the conditions of the stream may vary and temporarily (or all the way to solution) bring you in the two phase zone and cause the inconsistency.
Do not forget to look for over specifications of composition. Go to the workbook on the compositions tab. If you do not have a compositions tab, create one! A typical scenario is that you “reused” a stream that previously held a composition and forgot to erase that composition.
Using a mixer with the "Equalise all" setting (as you really should) and forgot to release the necessary pressures.

Some more elusive suspects:
If you mix streams where one flow is positive and another is negative, this holds the risk that the resulting stream will have some components with a positive flow and some with negative flows. The issue as such doesn't cause an instant inconsistency, it usually only shows up in the next separator.
A heat exchanger can sometimes be overspeciied and only flag an inconsistency the first time. After clicking the Go button, the inconsistency message may not re-appear during the next solve pass. But it will re-appear later! Normally the heat echanger itself would however flag the over specification by a yellow border around it in the pfd.