Using the wrong enclosure style
Guarani Giants are terrestrial. Young spiders may dig more, while larger animals often spend more time in the open, although individual Grammostola grossa can still choose to remain hidden. Build the enclosure around that lifestyle instead of using the same shape for every tarantula.
For Grammostola grossa, adult size is about 17–20 cm (6.5–8 in), but enclosure size should still follow the spider's current size. A sling does not benefit from being placed straight into a large adult setup.
Getting moisture wrong
For Guarani Giants, keep the enclosure mostly dry, with with a water dish and a slight moisture gradient for younger spiders. The mistake is usually treating the whole enclosure as either permanently wet or permanently dry instead of watching how the substrate actually behaves.
Keep fresh water available to Grammostola grossa. Persistent condensation, a stale smell or wet substrate that never dries are reasons to reduce water or improve airflow.
Overfeeding or chasing a schedule
Feed Guarani Giants according to size, appetite and abdomen, not because a calendar says food is due. Grammostola grossa can refuse meals for normal reasons, especially around premoult.
Do not keep adding prey when a Guarani Giant is already very well fed. Remove ignored live feeders and offer food again later.
Disturbing premoult or moulting
When Grammostola grossa shows premoult signs, remove live prey and leave the spider alone. A closed retreat is not an invitation to open it and check what is happening.
After a moult, wait until a Guarani Giant has hardened and the fangs have darkened before feeding. Larger spiders usually need longer than tiny slings.
Making maintenance too difficult
A good Grammostola grossa enclosure lets you refill water and remove food remains without dismantling the main retreat. If every job requires moving cork, digging into substrate or working directly over the spider, change the layout.
Handling is not part of routine care for Guarani Giants. Use long tools when useful and have a catch cup ready for rehousing.
Changing too much at once
Once a Guarani Giant has settled, avoid repeatedly rearranging the enclosure for appearance. Stable housing usually matters more to Grammostola grossa than a constantly changing display.
If something needs correcting, change one clear husbandry problem at a time. That makes it easier to see whether Grammostola grossa settles, feeds and uses the enclosure normally afterward.