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If you've ever experienced butter or corn flavors in beer, it's most likely due to the production of diacetyl and/or DMS (dimethyl sulfide).  Both of these are common off-flavors, and they are generally undesirable. 

Diacetyl

Diacetyl is commonly described as having an artificial butter flavor which leaves your mouth feeling slick with oil (it is acceptable in small amounts in some British styles). Every yeast produces diacetyl during fermentation in varying amounts, and bacteria also creates this compound. So, how can you prevent (or encourage) diacetyl?

• Temperature: towards the end of your fermentation you could raise the temperature of your brew by about 10°F. This encourages your yeast to metabolize and reabsorb the diacetyl compound.  Heat in bottles will also create diacetyl, so keep your bottles cool!
• Time: waiting a week to bottle/keg after your brew has hit final gravity will lessen your chance of diacetyl.
• Sanitation: remember, diacetyl can be produced by bacteria, so sanitize, sanitize, sanitize!

DMS (dimethyl sulfide)

DMS is described as vegetal scent, most commonly creamed corn.  It is a result of S-methylmethionine (SMM) breaking down.  SMM is produced during the germination portion of the malting process, but is driven away during kilning.  Lightly kilned malts (i.e. pilsner malts) retain much of the SMM.  Much like kilning, boiling your wort will help drive away the SMM.  Here's some quick tips to prevent DMS:

• Boil longer: if you increase your boil time to 90 minutes (specifically when using Pilsner malts), you drive away the SMM which produces DMS.
• DON'T COVER YOUR BOIL KETTLE: this will introduce condensation to your wort, which in turn will in turn convert SMM to DMS
• Cool your wort quickly: SMM converts to DMS due to HEAT, but is broken down by the boil.  In turn, if you go from boiling to hot and maintain it for an extended amount of time, the bonds can reform producing DMS.
• Choose different malts: Pilsner malts can contain as much as 8 times the amount of SMM as Standard 2-Row.  If you're truly worried about DMS, consider replacing a portion of your pilsner malt with a pale malt.