Medical implants of all kind must be tested for RF-induced heating to determine their MR safety and compatibility. Test conditions concerning E-field homogeneity and E-field drift during assessment time are defined in ISO/TS 10974 and ASTM-F2182. This study evaluates the influence of E-field drift and E-field homogeneity on RF-heating. It shows that both effects can lead to a temperature underestimation. Even for small implants whose volume is in a homogenous area within ±1dB, this underestimation can occur. System instability and implant size and position can further increase the underestimation of RF-induced heating and should be carefully considered.