A computer system, comprising: one or more processors; and one or more computer-readable media having stored thereon computer-executable instructions that are executable by the one or more processors to configure the computer system to replay a trace that relies on processor undefined behavior, the computer-executable instructions including instructions that are executable to configure the computer system to perform at least the following: identify reliance on processor undefined behavior by an instruction executed based on replay of traced program execution from a trace file; wherein identifying reliance on processor undefined behavior by the instruction executed based on replay of traced program execution from the trace file comprises: identifying execution of a prior instruction based on the replay of the traced program execution from a trace file, the prior instruction causing undefined processor state; and based on tracking the undefined processor state, identifying reliance on the undefined processor state by the instruction executed based on replay of traced program execution from a trace file, wherein tracking the undefined processor state includes tracking execution of subsequent instructions to determine if that state becomes defined or if it is relied upon; and based on the reliance on the processor undefined behavior, perform one or more of: initiate a notification of the reliance on the processor undefined behavior; skip to a key frame in the trace file, and resume replay at the key frame; fork replay using two or more potential behaviors; or continue replay using a selected behavior that is selected from among the two or more potential behaviors. one or more processors; and one or more computer-readable media having stored thereon computer-executable instructions that are executable by the one or more processors to configure the computer system to replay a trace that relies on processor undefined behavior, the computer-executable instructions including instructions that are executable to configure the computer system to perform at least the following: identify reliance on processor undefined behavior by an instruction executed based on replay of traced program execution from a trace file; wherein identifying reliance on processor undefined behavior by the instruction executed based on replay of traced program execution from the trace file comprises: identifying execution of a prior instruction based on the replay of the traced program execution from a trace file, the prior instruction causing undefined processor state; and based on tracking the undefined processor state, identifying reliance on the undefined processor state by the instruction executed based on replay of traced program execution from a trace file, wherein tracking the undefined processor state includes tracking execution of subsequent instructions to determine if that state becomes defined or if it is relied upon; and based on the reliance on the processor undefined behavior, perform one or more of: initiate a notification of the reliance on the processor undefined behavior; skip to a key frame in the trace file, and resume replay at the key frame; fork replay using two or more potential behaviors; or continue replay using a selected behavior that is selected from among the two or more potential behaviors. identifying execution of a prior instruction based on the replay of the traced program execution from a trace file, the prior instruction causing undefined processor state; and based on tracking the undefined processor state, identifying reliance on the undefined processor state by the instruction executed based on replay of traced program execution from a trace file, wherein tracking the undefined processor state includes tracking execution of subsequent instructions to determine if that state becomes defined or if it is relied upon; and based on the reliance on the processor undefined behavior, perform one or more of: initiate a notification of the reliance on the processor undefined behavior; skip to a key frame in the trace file, and resume replay at the key frame; fork replay using two or more potential behaviors; or continue replay using a selected behavior that is selected from among the two or more potential behaviors. initiate a notification of the reliance on the processor undefined behavior; skip to a key frame in the trace file, and resume replay at the key frame; fork replay using two or more potential behaviors; or continue replay using a selected behavior that is selected from among the two or more potential behaviors.