Nobody sets out to lose track of inventory. It happens gradually. A count is right in January, roughly right in March, and by June somebody is walking to the stockroom to look with their own eyes because the number on the screen has stopped meaning anything.
The useful part is that the drift is almost never random. It comes from a handful of specific, findable causes, and most of them are process problems rather than software problems.
Manual entry is the biggest single source
Every count typed by a human is a chance to fat-finger a digit. Transposing 43 into 34 is a nine-unit error that looks perfectly plausible on a report, which is exactly what makes it dangerous. An obviously wrong number gets investigated. A slightly wrong one gets trusted.
Barcode scanning removes the failure mode rather than reducing it. The label carries the identifier, the scanner reads it, and nobody is trying to remember which of two similar part numbers they are holding.
Movements nobody records
The second source is stock that moves without a transaction. Somebody grabs two units for an urgent job and means to log it later. A part gets borrowed by another department. A box is opened to check a color and never resealed or recorded.
These are friction failures rather than discipline failures. If logging a movement takes four screens and a login, people will do it afterwards or not at all. If it takes one scan, they will do it while they are standing there.
Units of measure that mean two things
You buy in cases and issue in units. Somebody records a case as a unit, or the reverse, and the count is now wrong by the case size.
This one compounds badly, because the error is large enough to notice but the cause is invisible on the record. It looks like theft or loss when it is arithmetic.
Returns and reversals
Anything coming back into stock is a second chance to get it wrong. A canceled order, a part returned unused, a job that used less than planned. If the return path is less well defined than the issue path, and it usually is, that asymmetry is where error accumulates.
How to find your own drift
Pick one item with a count you do not trust and reconstruct its history. Every receipt, every issue, every adjustment. Somewhere in that list is the shape of your problem, and it is normally the same shape for everything else.
Then look at what it costs a person to do the right thing. If recording a movement correctly is slower than skipping it, the count will keep drifting no matter what policy gets written.
Counting a few items regularly beats one annual count that goes badly. Small, frequent checks catch drift while the cause is recent enough to identify.
Where software helps, and where it does not
Software cannot make people log movements. What it can do is make logging faster than not logging: scannable labels, locations that match where things physically are, reorder points that trigger without anybody watching a report, and kits so items that travel together are counted together.
Knowledge ERP exists because of this problem. Its first customer was a college tracking consumables and calibrated equipment for a medical laboratory training program, which is an unforgiving place to have a count you cannot trust.
If your counts have started drifting, the fix is usually not a stricter policy. It is removing the friction that made the shortcut attractive.