Since device has no battery, time is kept only by monitoring voltage decay on a big capacitor. Major downfall is the fact that such timing is imprecise and doesn't allow for a big time spans. In practice TmpUsb can deal with up to 10 seconds of delay.
Since most operating systems cannot really format such a small drive and such attempt might possibly damage logical structure, decision has been made to check MBR and boot sector (including partition table) areas on every boot. Effect of this verification is that any change to verified areas is not possible and drive will self-erase if such change is noted.
Chip used for TmpUsb has flash specified for at least 10000 write cycles. Since there is no block remapping as it is custom on the modern flash drives, this roughly means that you can safely write to this drive approximately 5000 times (it does suffer from high write amplification). While it will probably handle a bit more, this is definitely not the device when constant writing is expected.
Total size of this drive is 11 KB. However, this is not a size user will have available. Six sectors (3 KB) are immediately taken by mandatory MBR and boot sector, followed by FAT and a root directory entries. OS only gets to see 8 KB. Once drive is plugged-in, some Windows will add its System Volume Information folder on each drive it sees and that means that it gets hit with additional 1 KB penalty and at least four directory entries are gone. Under Windows available size is thus 7-8 KB (depending on version) while we get 8 KB under Linux.