In all current D6 games, you must at roll a wild die with every skill check. For example, if you have a skill check of 1D, that die is the wild die. If you have 6D, one is the wild die and five are regular.
Anyway, the wild die counts toward your regular total on all rolls.
However if it rolls a 1 or a 6 it will have different effects.
If you roll a 6, you roll that die again and add both rolls to the final total. If you roll another 6 on the wild die, you roll it again and again until you stop rolling 6s. This can create for some massively (read heroic-level) skill successes. Typically, the GM is asked to embellish the effect in a positive direction. For example, if you get a skill total of 32 (with two 6 rolls on the wild die) on a computer search roll, you not only find the information on the crook you're researching, but you also find out that he's also the cousin of the city mayor, the two own a business together, and they have several shadey dealings going on.
If you roll a 1, you trigger a critical failure. Not only does that one count toward your total, you also loose the highest roll you made on that skill attempt, further lessening your success.
If you roll a 1, but still succeed at the skill attempt, you do something embarassing or foolish or something to lessen the success. For example, you roll a 1 on the wild die, but still pass your Repair skill check. Sure you succeed, but you break your hydrospanner in the process, meaning you can't make anymore.
If you roll a 1 and fail at the attempt, you really botch the job. For example, under the same skill attempt above, you fail your Repair attemp, drop your hydrospanner down into the sublight drive engine, and do even more damage to your engine than before.