For users of IB 5.6 and 6.01 and Firebird 1 beta, you need to be
aware of this bug. In short, backup and restore before the
applicable transaction-count maximum is reached!

1024 byte pages 131,596,287.
2048 byte pages 265,814,016.
4096 byte pages 534,249,472.
8192 byte pages 1,071,120,384.

Details here:

from David Robinson
http://www.howtodothings.com/showarticle.asp?article=443

more from Ann H.
http://www.howtodothings.com/showarticle.asp?article=442

Regards,
Andrew Ferguson