A series of conversations from several days...compacted into one nightmare. Enjoy:
Front line dude: Acme has a drive space issue...can I shrink the log file?
Kevin3NF: Give it a shot (this is SOP for Acme on this drive)
FLD: Didn't work
Kevin3NF: Give me the ticket, I'll take a look.
Kevin3NF: Hey ACME developer...you have a 2 day old transaction taking up all your T-log space...
Acme: ok...let me truncate the log
Kevin3NF: WAIT A SECOND!
Acme: Did that help?
Kevin3NF: No. You just invalidated the T-log backup stream and started causing the backup to fail
Kevin3NF: Cuz that big open transaction is still there, and SQL Server thinks there is no full backup now.
Kevin3NF: (Sees pictures of crickets in his Inbox)
Acme: (hours later): go ahead and run a full backup tomorrow afternoon
Kevin3NF: You know you don't have a valid backup to recover to, right?
Acme: Yes, but we can't backup now...
Acme: Hey...who told you to run a full backup?!?!?
Kevin3NF: You did...I have the email.
Acme: (complaint to Kevin3NF's bosses)
Time passes...repeat scenario.
Big transaction, full log file, fill drive, lather, rinse, repeat.
Customer: FAIL. >:(
Acme me owes ME some fries...but I get paid to deal with stuff like this every day :)