Bug

Bugs, introduced in MegaMan Battle Network 3 along with Navi Customizer, are glitches in a NetNavi's body, but only MegaMan.EXE he can utilize and use its power. These glitches may cause a variety of side-effects during a battle. Many things can cause a bug, but usually:
 * Break a Navi Customizer rule: If the NaviCust is not used properly, then MegaMan can have a bug. The bug depends on which NaviCust part was used to break a rule.
 * Use of Bug Style or Hub.BAT: The Bug Style, as its name implies, causes many bugs. The bugs of this style can either be good or bad depending on just luck. The Hub.BAT (which, during Battle Network 2 can only be accessed by using Hub Style) imitates every style from BN2 with its abilities, making it the best style (or the best NaviCust part in Battle Network 3 - 6), but causes an HP bug that halves MegaMan's HP. This side-effect was no longer present in the Hub.BAT during Battle Network 6.
 * Get hit by a Bug Bomb: In Battle Network 6, there is a "PvP only" Battle Chip that can bug the other player. Effects are random, but unlike the Bug Style, it can only cause negative effects.

Bug Fragment
A Bug Fragment (バグのかけら), Bug Frag for short, is a accumlation of bugs, a type of junk data found in the MegaMan Battle Network series.

In MegaMan Battle Network 2 the leader of Gospel, Sean, was trying to clone Bass.EXE though the use of Bug Frags. This however went terribly wrong and the Bass clone ended up mutating into the Gospel multibug organism that started overloading the servers with radiation to almost 600%. MegaMan and Lan end up defeating Gospel. Later, in the Secret Area scenario of MegaMan Battle Network 3, Bass eventually finds Gospel and fuses with him, after which he survives by absorbing Bug Frags that the player has deposited in the Bug Frag Trader. After 300 Bug Frags are deposited, he shows up and challenges MegaMan the next time MegaMan enters the Secret Area.

Additionally, starting with the second game, and to the end of the series, Bug Frags are also used as an alternate form of currency, as various 'Bug Frag Traders' appear in the game. Chips and programs sold by these traders are usually rarer in value than those sold by standard merchants, though they are often in an out-of-the-way location.

In the second game, Bug Frags were only obtainable through Blue Mystery Data, and only a total of 16 (or so) are obtainable in the game without usage of any glitches.

In the third game, finishing off viruses or bosses with a counter hit from any attack would also yield Bug Frags, and this is indicated by a star after the Busting rank display. One star (countering 1 virus) yields 1 Bug Frag, 2 (from 2 viruses) yields 3, 3 (from 3 viruses) yields 8. Finishing a boss off with a counter hit will yield 10 Bug Frags. While this means of obtaining Bug Frags is highly exploitable with an easily-countered boss, the Bug Frags serve multiple purposes, like boosting the strength of his virus-summoning chips, getting past certain barriers or even unlocking another boss through the Bug Frag Trader.

In the fourth game onwards, Bug Frags can only be obtained through Green Mystery Data and in-battle Mystery Data. Green Mystery Data collected outside battle always offers one Bug Frag, while in-battle Mystery Data gives a varying amount depending on the area. In the fourth game, where starting a new playthrough causes all Blue and Purple Mystery Data to regenerate with different content, all Blue and Purple Mystery Data will yield Bug Frags after all of their contents from the first 3 playthroughs have been collected.