I find it easy to forget that the “Guideline” element in a ConstraintLayout requires a value from 0 to 1 for its “layout_constraintGuide_percent” attribute:

<android.support.constraint.Guideline 
  android:id="@+id/center_guideline" 
  android:layout_width="wrap_content" 
  android:layout_height="wrap_content" 
  android:orientation="vertical" 
  android:visibility="visible" 
  app:layout_constraintGuide_percent="0.50" />

Don’t people usually think of numbers between 0 and 100 when they hear the word “percent”?

Android Studio 3.1.2’s layout designer does not seem to warn or complain if you use a value above 1 (e.g. “50”). But when I did that, the layout behaved unexpectedly.