The eval command is deprecated and a warning message is shown per connection. As the mongo shell shows the warnings from the startup log, when opening a mongo shell it will show the same repeated warning message for each connection, taking all the window space:

2016-04-05T13:21:32.105+0000 W COMMAND  [conn712600] the eval command is deprecated
2016-04-07T03:21:30.126+0000 W COMMAND  [conn1010265] the eval command is deprecated

It might be useful to have just a single warning when using eval instead of one per connection just repeating the message.