We upgraded to 11.6.1 last weekend and seeing very interesting stuff while doing

prorest $DBNAME $TAPEDRIVE -verbose

Start of restoring the target DB... (9433)

restore: 15:55:38 Estimate 0 hours 1 minutes more processing 838 Percent complete.

restore: 15:56:38 Estimate 0 hours 1 minutes more processing 2019 Percent complete.

...

restore: 16:42:38 Estimate 0 hours -46 minutes more processing -6000 Percent complete.

restore: 16:43:38 Estimate 0 hours -42 minutes more processing -4512 Percent complete.

Go figure how much time is left :)

IBM AIX OE 11.6.1