Sketches should be fully constrained
Why leaving geometry free creates problems much later.
Sketches should be fully constrained because a sketch with any geometry still free to move looks correct only in the exact position it happens to be sitting in right now, and that hidden freedom stays invisible until some later change nudges the sketch into a different, unintended shape. It is one of the first habits worth teaching a beginner, precisely because it is one of the easiest to skip without any immediate consequence.
Degrees of freedom, removed one at a time
A sketch is built from lines, arcs and points that a constraint system pins down using dimensions and geometric rules: this length is exactly this value, this line is exactly horizontal, this point sits exactly on that other line. Each constraint removes one degree of freedom, one direction the geometry could otherwise have shifted in, and a sketch is only fixed once every degree of freedom has been removed.
A rectangle shows the bookkeeping. Its four corners start with eight degrees of freedom between them, two each. Making two sides horizontal and two vertical removes four, and the shape still needs a width, a height and a position (two dimensions to the origin, or one corner fixed on it) before it stops moving. Leave off the height and the rectangle still looks finished on screen, sitting at whatever height it was drawn.
That is how a correct-looking sketch hides freedom: a line whose length happens to match what was intended without anything fixing it there, a point that sits on another line by coincidence. Nothing has yet forced that freedom to move, and the software gives no warning until the geometry is dragged or a later change shifts something the free geometry was relying on. By then the sketch may sit several features back in the model's history, rarely viewed, which is why the resulting shift so often looks inexplicable.
A camping chair with one loose bolt
A folding camping chair with every joint pinned and every strut tensioned sits rigid however it is picked up, folded or set down. The same chair with a single bolt left loose can look and sit exactly like the properly assembled one until it is lifted at an angle it has not met before, when the loose joint lets the whole frame shift or sag in a way nobody watching it stand still would have predicted. It only appeared rigid because nothing had tested the one degree of freedom the loose bolt left behind. Skipping the bolt saved nothing worth having on the day the chair was put together, and the whole cost arrived later, at a moment nobody chose.
An underconstrained sketch hides the same kind of untested freedom behind an appearance of being fine. Nobody assembling the chair tests every angle it might one day be lifted from, and nobody building a sketch checks every way a later change might nudge it, which is why the constraint itself has to be relied on instead of a visual check.
Trusting the colour over the shape
Most CAD software shows fully constrained geometry in a distinct colour from underconstrained geometry, because a glance at the shape is an unreliable way to catch the problem. A sketch of twenty or thirty pieces of geometry needs only one missing dimension among dozens of correct ones to carry hidden freedom, and looking at that colour before leaving a sketch is one of the cheapest habits a beginner can pick up. It prevents a much longer investigation later, tracing back through a model to find which long-forgotten sketch caused an unexpected change of shape.
A model built entirely from fully constrained sketches behaves predictably under every later edit, since nothing in it can move except through a dimension changed on purpose. A model with a few underconstrained sketches scattered through it carries a standing risk that any edit might wake up a piece of geometry nobody remembered was never pinned down.
Rough sketches while the shape is still unsettled
Full constraint is not always the right immediate goal while a sketch is still being roughed out, since dimensioning every line before the overall shape is settled wastes effort on geometry that may be deleted or redrawn. Many experienced users deliberately work through a loosely constrained stage, dragging geometry freely to explore proportions, and lock the sketch down once the shape is decided. A good finishing routine is to confirm the colour indicator shows full constraint and to read through the dimensions, making sure each one is a value someone meant to set and not a number the software filled in from wherever the line happened to land.
The discipline that matters is finishing that process and treating a sketch as done only once it reaches full constraint. A rough sketch mid-exploration is a normal and useful stage. An underconstrained sketch left behind in a finished model causes the trouble, and the difference comes down to whether anyone went back to close off the freedom before moving on to the next feature.