Every joint you've cut so far passed inspection on the bench. Tenon closed by hand pressure. Dovetails registered without a mallet. Dado accepted its shelf with a satisfying snick.
None of that tells you whether they'll behave when you put them all together at once.
What the bench hides
A tenon that fits snug on its own may have drifted a thirty-second while you were cutting the fifth one. A dado that holds its shelf perfectly won't mention that it's pulling the case into wind until all four dadoes are assembled and you lay winding sticks across the top.
This is the tolerance cascade. Small deviations compound. A sixteenth of variation in shoulder-to-shoulder distance across four rails creates a racking problem you can measure with a tape but couldn't see in any single joint. Two dadoes cut a thirty-second off the same layout line put a shelf in crooked. Each joint passes. The assembly doesn't.
The only way to find a cascade is to assemble the thing.
What a dry run actually is
Full assembly. Every part, every joint, every clamp at real pressure. Not finger-tight. Not approximate. The same clamps you'll use during glue-up, in the same positions, with the same cauls if you're using cauls. The same sequence you'll follow when the clock is running.
No shortcuts. A shortcut now buys you a surprise later, and surprises during glue-up cost furniture.
Sequence matters more than you think
Assemble in the exact order you'll glue. This sounds obvious. It gets skipped more than any other step.
The reason is the third-hand problem. The joint that goes in second may need to be supported while the joint that goes in third is driven home. Discovering this with wet joints and a running clock is exactly the kind of thing the dry run exists to prevent. Figure out where you need a helper, a standoff, or a creative clamping solution now, while you can stop and think about it.
What to check
Shoulder fit. Every shoulder should close completely at the reference face. No strip of light at any corner. If one corner is open, the shoulder is short of the knife wall or the mating face isn't flat. Chalk the mating face, close under clamp pressure, open and read where contact happened. The chalk tells you what the eye can't.
Square. Any rectangular assembly: tape-measure corner-to-corner both directions. Equal diagonals mean square. Unequal means racking. If it's slight, skew the clamp line toward the long diagonal and recheck. If it's significant, the problem is in the joint geometry, not the clamp placement. Go find it.
Panel freedom. Any floating panel in a frame must be completely free under full clamp pressure. If you feel it bottom out, the groove is too shallow or the panel is oversized. Fix it now.
Hardware openings. If the build has drawers, slides, or hardware with registered positions, confirm the openings match the hardware dimensions during this assembly. Not after.
Mark everything
Every problem gets a pencil mark directly on the part. Not a mental note. Not a notebook entry.
You will be under time pressure during glue-up. You will not remember that the fourth tenon on the right rail had a slight gap at the near corner. The pencil mark remembers for you. Mark it, fix the joint, cross the mark off.
Run it again if you had to fix something
If you corrected anything significant, a shoulder that needed the shoulder plane, a tenon that needed the router plane, assemble it again. It doesn't have to be a full re-clamp if the fix was isolated. But if you reworked more than one joint, or one critical joint, run the whole thing.
The cost is twenty minutes. The alternative is discovering that your correction created a secondary problem with wet glue in the joint and thirty seconds of open time remaining.
When good enough is good enough
The dry run is a go/no-go check, not a perfection audit. The standard: closes under clamp pressure, no gaps at show-face shoulders, square diagonals, no bottomed-out panels.
A micro-gap on a secondary face, the inside of a rail-to-leg joint on a table that lives against a wall, may not be worth the correction risk if the show-face shoulder closes completely. That's a judgment call.
The show-face shoulder gap is not a judgment call. Close it.