The four that catch people out:
- Validity starting at purchase instead of activation. Some providers start the clock when you pay. Check that the period starts on first network connection.
- Throttling after a "fair use" threshold on unlimited plans. Unlimited often means full speed to a cap, then 512 kbps or slower.
- Coverage that excludes part of a region. A plan named after an island or a region may cover only part of it politically or technically.
- Top-ups priced above the original plan. Running out mid-trip can cost more per gigabyte than the first purchase.
None of these appear on the price tag. They are worth checking before buying, whoever you buy from.