Some Things to Watch out for:
Contamination of beer can happen at any stage of the brewing process. Some are not readily apparent. But any problem that can be easily drank will not cause physical harm. A few infections that may cause severe gastric distress will first be noted by their appalling smell. Here are some warning signs:
Mold floating on top of the fermenting beer. Toss it.
The beer has slimy strands in it. This is a sure sign of Lacto infection. Toss it.
The bottled beer has a milky layer at the top and/or small residue bumps clinging to the sides of the bottle neck in the airspace. This is a micro-derm infection. The beer will smell rotten and taste nasty. Do not confuse this with the dew that condenses near the bottle cap; the dew is normal. Also, Priming with DME will leave a protein ring around the top of the bottle, just like what is left on the sides of the fermenter. This is also normal.
The bottled beer has a very sweet smell, like molasses.
This is a sign of an Aceto (acetic) infection. The beer is on its way to turning into malt vinegar. Malt vinegar is good, but not what was intended. The bottled beers are getting worse with time, a stale, cardboard-like or sherry-like flavor is becoming noticeable. This is a symptom of oxidation. Drink the beers sooner and try to avoid splashing the hot wort next time.
A skunk-like or cat-musk smell. The beer is light struck. Always store beer in a dark or shaded area.